Dispersion-compensating optical fiber and hybrid transmission line

ABSTRACT

In order to provide a dispersion-compensating optical fiber able to be applied over a broad wavelength band, having a large effective area, and as a result, suppressing the occurrence of non-linear effects, the present invention comprises a dispersion-compensating optical fiber that compensates chromatic dispersion of a 1.3 μm single-mode optical fiber over the entire wavelength range of 1.53-1.63 μm characterized in that, chromatic dispersion at a wavelength of 1.55 μm is −50 ps/nm/km or less, the dispersion slope is negative over the entire wavelength range of 1.53-1.63 μm, a cutoff wavelength is provided at which there is substantially single-mode propagation, bending loss is 30 dB/m or less, effective area is 20 μm 2  or more, and the absolute value of chromatic dispersion during compensation of the chromatic dispersion of a 1.3 μm single-mode optical fiber serving as the target of compensation is 0.5 ps/nm/km or less.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a dispersion-compensatingoptical fiber and an optical transmission path. The presentspecification is based on patent applications filed in Japan (JapanesePatent Application No. 2000-54646, Japanese Patent Application No.2000-159071, Japanese Patent Application No. 2000-216587, JapanesePatent Application No. 2000-241547 and Japanese Patent Application No.2000-266169), and the contents described in said Japanese patentapplications are incorporated as a part of the present specification.

[0003] 2. Background Art

[0004] Optical communication systems that transmit at the 1.55 μmwavelength band (the so-called C-band: typically covering a range ofabout 1.53-1.57 μm) are used practically by combining single-modeoptical fibers for transmission such as a “1.3 μm band dispersionsingle-mode optical fiber having nearly zero chromatic dispersion at awavelength of 1.3 μm” or a “standard single-mode optical fiber”, anddispersion-compensating optical fibers.

[0005] For example, since the chromatic dispersion of a 1.3 μmsingle-mode optical fiber is roughly +17 ps/nm/km (positive chromaticdispersion) at a wavelength of 1.55 μm, when this is used to performoptical communications in the 1.55 μm wavelength band, considerablechromatic dispersion occurs. In contrast, since dispersion-compensatingoptical fibers have negative chromatic dispersion in which the absolutevalue in the 1.55 μm wavelength band is comparatively large, bycombining these in the manner described above, the chromatic dispersionthat occurs in ordinary 1.3 μm single-mode optical fibers extending, forexample, for several tens of kilometers, can be compensated by adispersion-compensating optical fiber having a comparatively short usedlength.

[0006] In addition, since the dispersion slope of a 1.3 μm single-modeoptical fiber in the 1.55 μm wavelength band is roughly +0.06 ps/nm²/km(positive value), in order to compensate according to this dispersionslope with the chromatic dispersion, it is preferable to use adispersion-compensating optical fiber having a negative dispersionslope. If dispersion slope can be compensated, this can also be used inapplications involving transmission of a plurality of pulsed lighthaving different wavelengths as in the manner of high-density wavelengthdivision multiplexing transmission (DWDM transmission).

[0007] On the other hand, transmission characteristics deteriorate whennon-linear effects occur in optical fibers. In the case of propagatinghigh-power signal light in the manner of optical communication systemsusing wavelength division multiplexing transmission and an opticalamplifier that have already been used practically in particular, due tothe high power density, non-linear effects tend to occur easily,resulting in the need for technology that suppress non-linear effects.

[0008] Although methods for suppressing non-linear effects have beenproposed, including a method in which the non-linear refractive index ofthe optical fiber is decreased by reducing the amount of Ge, F or otherdopant doped to the core, and a method in which Brillouin scattering,which is one of the non-linear effects, is suppressed by changing theouter diameter of the optical fiber when drawing from the fiber basematerial, enlargement of the effective area (which may be abbreviated asAeff) of the optical fiber is a particularly effective method.

[0009] However, in the above-mentioned dispersion-compensating opticalfibers of the prior art, although such optical fibers have beendeveloped which attempt to improve the so-called performance index (FOM;Figure of Merit), which indicates the amount of chromatic dispersion perunit loss, while also being able to compensate the dispersion slope, ithas been difficult to simultaneously realize these characteristics alongwith enlargement of Aeff.

[0010] In addition, a system that performs optical communication in theso-called L-band (1.57-1.63 μm), which is of a longer wavelength thanthe C-band that has been used in the past, has recently been examined.Wavelength bands used for optical transmission at present or in thefuture are reaching or will reach a broad range of 1.45-1.63 μm, whichincludes the so-called S-band (1.45-1.53 μm).

[0011] Thus, although a dispersion-compensating optical fiber isrequired that is able to compensate the chromatic dispersion anddispersion slope of single-mode optical fibers for transmission in notonly the C-band, but also other wavelength bands such as the S-band andL-band, conventional dispersion-compensating optical fibers have beenunable to adequately satisfy this requirement. Consequently, theseoptical fibers have been inadequate particularly in applications towavelength multiplexing, high-speed, long-distance transmission and soforth.

[0012] In addition, although large Aeff is also simultaneously requiredin these wavelength bands, conventional dispersion-compensating opticalfibers were unable to accommodate this requirement as well.

[0013] Moreover, dispersion-compensated optical fibers are required tohave single-mode propagation at the used wavelength band and bendingloss that is small enough to allow practical use.

[0014] In addition, these dispersion-compensating optical fibers of thedispersion slope-compensating type have conventionally been incorporatedin optical communication systems in the form of modules by being housedin, for example, a suitable case. Recently however, studies have beenconducted that attempt to form the dispersion-compensating optical fiberitself into a cable and insert it into the transmission path, andseveral of these attempts have been reported.

[0015] This is because, if a dispersion-compensating optical fiberitself was able to be used as a transmission path, since it would bepossible to eliminate the arranging space of the module, while alsobeing able to substantially shorten the length of the optical fiberthrough which the optical signals are transmitted, the transmissioncharacteristics of the overall system could be improved.

[0016] However, although reports have been made regardingdispersion-compensating optical fibers of the dispersionslope-compensating type that place the emphasis on compensation ofchromatic dispersion and dispersion slope as well as reduction of loss,there have been no effective studies or reports made regardingenlargement of Aeff.

[0017] Suppression of non-linear effects is essential for achieving thefaster speeds, longer distances and wavelength multiplexing describedabove, and in the case of inserting a dispersion-compensating opticalfiber of the dispersion slope-compensating type in the form of atransmission path, there are cases in which it is difficult to achievepractical use unless Aeff is provided to an extent that is able toeffectively suppress non-linear effects.

[0018] In addition, dispersion-compensating optical fibers of the priorart required that, for example, the refractive index of the core centerbe larger than that of an ordinary single-mode optical fiber, and hadthe problem of increasing the amount of dopant doped to the core center.

[0019] Normally, a core center is formed from quartz glass doped with adopant such as germanium etc. that provides the action of increasingrefractive index, while the cladding provided around an outer peripheryof the core is formed from pure quartz glass or fluorine-doped quartzglass, etc.

[0020] In addition, the glass transition point of the quartz glassdecreases proportional to the amount of dopant doped. Thus, if theamount of dopant doped increases, since the difference in viscosities ofthe core and cladding increases when the fiber base material is heatedand melted to draw the optical fiber, the drawing rate and drawingtemperature are restricted from the viewpoint of mechanical strength,thereby resulting in the problem of being unable to obtain adispersion-compensating optical fiber with low loss.

[0021] In addition, if the refractive index of the core center is high,Aeff tends to decrease. This results in greater susceptibility to theoccurrence of non-linear effects, thereby leading to the problem ofdeterioration of transmission characteristics.

[0022] However, in the dispersion-compensating optical fibers of theprior art, optical fibers realizing chromatic dispersion and dispersionslope compensating effects while enabling the refractive index of thecore center to be comparatively small have been unable to be obtained,resulting in problems in terms of ease of production, low loss andnon-linearity, etc.

[0023] In addition, if Aeff is enlarged in an example of adispersion-compensating optical fiber of the prior art, the absolutevalue of chromatic dispersion tends to become smaller, resulting in theproblem of the length for compensating single-mode optical fibers fortransmission becoming excessively long.

SUMMARY OF THE INVENTION

[0024] The object of the present invention is to provide adispersion-compensating optical fiber that can be applied to a widewavelength band, has a large Aeff, and as a result, is resistant to theoccurrence of non-linear effects.

[0025] Moreover, the object of the present invention is to achieve atleast one of the following first through fifth objects after havingachieved this object.

[0026] A first object of the present invention is to provide adispersion-compensating optical fiber that is able to guaranteechromatic dispersion of a 1.3 μm single-mode optical fiber over theentire wavelength range of 1.53-1.63 μm, while also being able toguarantee single-mode propagation and have small bending loss.

[0027] Moreover, another object is to provide a dispersion-compensatingoptical fiber that is able to simultaneously compensate dispersionslope.

[0028] Moreover, another object is to provide a dispersion-compensatingoptical fiber having a large Aeff that is able to suppress non-lineareffects.

[0029] A second object of the present invention is to provide adispersion-compensating optical fiber of the dispersionslope-compensating type that is able to realize low loss and lownon-linearity while maintaining the inherent function of compensatingchromatic dispersion and dispersion slope.

[0030] More specifically, the object is to provide adispersion-compensating optical fiber provided with a large Aeff inorder to realize low non-linearity.

[0031] A third object of the present invention is to provide adispersion-compensating optical fiber having large Aeff and low loss.

[0032] In addition, an object is to provide a dispersion-compensatingoptical fiber for which the difference in viscosities between the coreand cladding during drawing is small.

[0033] More specifically, the object is to provide adispersion-compensating optical fiber that is able to make the relativerefractive index difference based on the cladding of the highest layerof the core comparatively small, while also decreasing the amount ofdopant doped to this layer.

[0034] A fourth object of the present invention is to provide atechnology for obtaining a dispersion-compensating optical fiber of thedispersion slope-compensating type having low loss. In addition, anobject is to lower bending loss independent of the used wavelength band,without significantly impairing other characteristics, in comparisonwith dispersion-compensating optical fibers of the prior art.

[0035] Moreover, an object is to realize a dispersion-compensatingoptical fiber suitable for long-distance transmission that is able tosuppress the chromatic dispersion value and dispersion slope value at aspecific length.

[0036] A fifth object of the present invention is to provide adispersion-compensating optical fiber that is able to compensatechromatic dispersion of a single-mode optical fiber for transmissionover a wide wavelength band.

[0037] In addition, an object is to provide a dispersion-compensatingoptical fiber that is able to enlarge Aeff and suppress non-lineareffects.

[0038] Moreover, an object is to provide a dispersion-compensatingoptical fiber in which the length required for compensating single-modeoptical fibers for transmission, while preventing the absolute value ofchromatic dispersion from becoming small even if Aeff is enlarged, iscomparatively short.

[0039] In order to achieve the above first object, thedispersion-compensating optical fiber of a first embodiment of thepresent invention is a dispersion-compensating optical fiber thatcompensates chromatic dispersion of a 1.3 μm single-mode optical fiberover the entire wavelength range of 1.53-1.63 μm wherein, chromaticdispersion at a wavelength of 1.55 μm is −50 ps/nm/km or less, thedispersion slope is negative over the entire wavelength range of1.53-1.63 μm, a cutoff wavelength is provided at which there issubstantially single-mode propagation over the entire wavelength rangeof 1.53-1.63 μm, bending loss is 30 dB/m or less over the entirewavelength range of 1.53-1.63 μm, Aeff is 20 μm² or more over the entirewavelength range of 1.53-1.63 μm, and the absolute value of chromaticdispersion during compensation of the chromatic dispersion of a 1.3 μmsingle-mode optical fiber serving as the target of compensation is 0.5ps/nm/km or less over the entire wavelength range of 1.53-1.63 μm.

[0040] In addition, in order to achieve the above second object, thedispersion-compensating optical fiber of a second embodiment of thepresent invention characterized in that, in a used wavelength bandselected from 1.53 to 1.63 μm, Aeff is 30 μm² or more, bending loss is40 dB/m or less, chromatic dispersion is −40 to −10 ps/nm/km, theabsolute value of chromatic dispersion over the entire transmission pathconnected to a single-mode optical fiber for transmission provided withpositive chromatic dispersion is 4.0 ps/nm/km or less, the absolutevalue of dispersion slope over the entire transmission path is 0.03ps/nm²/km or less, and a cutoff wavelength is provided that allowssubstantially single-mode propagation at the used length in the abovetransmission path.

[0041] In order to achieve the above third object, thedispersion-compensating optical fiber of a third embodiment of thepresent invention is characterized in that, a core and a claddingprovided around an outer periphery of said cladding are provided,

[0042] said core is provided with a central core portion having arefractive index higher than said cladding, an intermediate core portionprovided around an outer periphery of said central core portion having arefractive index lower than said cladding, and a ring core portionprovided around an outer periphery of said intermediate core portionhaving a refractive index higher than said cladding,

[0043] when radii and relative refractive index differences based on thecladding of the central core portion, the intermediate core portion andthe ring core portion are expressed as (a,Δ₁), (b,Δ₂) and (c,Δ₃),respectively,

[0044] a is 2-3 μm, Δ₁ is 0.9 to 1.5%, Δ₂ is −0.30 to −0.45%, Δ₃ is 0.2to 1.2%, b/a is 2.0 to 3.5, and c/a is 3.0 to 5.0,

[0045] in a used wavelength band selected from 1.53 to 1.63 μm, Aeff is20 μm² or more, bending loss is 40 dB/m or less, chromatic dispersion is−65 to −45 ps/nm/km, and a cutoff wavelength is provided that allowssubstantially single-mode propagation, and

[0046] the compensation rate of dispersion slope when compensating asingle-mode optical fiber, at a length at which chromatic dispersion ofsaid single-mode optical fiber having a zero dispersion wavelength at awavelength shorter than the above used wavelength band can becompensated to zero, is 80-120%.

[0047] In order to achieve the above fourth object, thedispersion-compensating optical fiber of a fourth embodiment of thepresent invention is characterized in that,

[0048] a core and cladding provided around an outer periphery of saidcore are provided,

[0049] said core is provided with a central core portion having arefractive index higher than said cladding, an intermediate core portionprovided around an outer periphery of said central core portion having arefractive index lower than said cladding, a ring core portion providedaround an outer periphery of said intermediate core portion having arefractive index higher than said cladding, and a side ring core portionprovided around an outer periphery of said ring core portion having arefractive index lower than said cladding,

[0050] in a used wavelength band selected from 1.45 to 1.63 μm,chromatic dispersion is −70 to −45 ps/nm/km, chromatic dispersion slopeis negative, Aeff is 20 μm² or more, and a cutoff wavelength is providedthat allows substantially single-mode propagation, and when asingle-mode optical fiber is compensated at a length at which chromaticdispersion of this single-mode optical fiber having zero dispersion at awavelength shorter than said used wavelength band can be compensated tozero, the compensation rate of dispersion slope defined as RDS(DCF)/RDS(single-mode optical fiber)×100, when the value obtained by dividing thedispersion slope of the single-mode optical fiber by chromaticdispersion of the single-mode optical fiber is taken to be RDS(single-mode optical fiber), and the value obtained by dividing thedispersion slope of the dispersion-compensating optical fiber bychromatic dispersion of the dispersion-compensating optical fiber istaken to be RDS (DCF), is 80-120%, and bending loss at a wavelength of1.63 μm is 50 dB/m or less.

[0051] Furthermore, RDS is the abbreviation of relative dispersionslope.

[0052] In order to achieve the above fifth object, thedispersion-compensating optical fiber as claimed in a fifth embodimentof the present invention is characterized in that, a core and a claddingprovided around an outer periphery of said core are provided,

[0053] said core is provided with a central core portion having arefractive index higher than said cladding, an intermediate core portionprovided around an outer periphery of said central core portion having arefractive index lower than said cladding, and a ring core portionprovided around an outer periphery of said intermediate core portionhaving a refractive index higher than said cladding,

[0054] at a wavelength of 1.55 μm, chromatic dispersion is −40 ps/nm/kmor less and −65 ps/nm/km or more, chromatic dispersion slope isnegative, Aeff is 18 μm² or more, bending loss is 50 dB/m or less, andthe cutoff wavelength allows substantially single-mode propagation, and

[0055] the chromatic dispersion of a hybrid transmission line combinedwith a single-mode optical fiber for transmission for which in 1.55 μm,Aeff is 40 μm² or more, chromatic dispersion is positive, and the cutoffwavelength allows substantially single-mode propagation is −0.5 ps/nm/kmor more and +0.5 ps/nm/km or less at a used wavelength band over acontinuous range of 0.06 μm or more selected from a wavelength range of1.45-1.63 μm.

[0056] In the first embodiment of the present invention, adispersion-compensating optical fiber can be provided that is able tocompensate chromatic dispersion and dispersion slope of a 1.3 μmsingle-mode optical fiber over the entire range of 1.53-1.63 μm andcompensate single-mode propagation while having low bending loss, largeAeff and is able to suppress non-linear effects.

[0057] In the second embodiment of the present invention, adispersion-compensating optical fiber of the dispersionslope-compensating type can be provided that is able to realize low lossand low non-linearity while maintaining its inherent function ofcompensating chromatic dispersion and dispersion slope.

[0058] In the third embodiment of the present invention, adispersion-compensating optical fiber having low loss can be obtained bydrawing at lower tension than that of the prior art since the relativerefractive index difference of a layer provided with the highestrefractive index of a core is small, and non-linear effects can beSuppressed by enlarging Aeff.

[0059] In addition, the dispersion-compensating optical fiber of a thirdembodiment of the present invention is able to construct a hybridtransmission line suitable for wavelength division multiplexingtransmission, long-distance transmission and so forth by combining witha single-mode optical fiber.

[0060] The dispersion-compensating optical fiber of the fourthembodiment of the present invention is able to reduce the difference insoftening temperature and hardening temperature between a central coreportion and a cladding, and reduce the difference in viscosity at itsdrawing temperature during drawing by forming the entire portion, andparticularly the cladding, from quartz glass containing dopant.

[0061] As a result, the stress remaining in the central core portion andso forth after drawing can be reduced, and even if drawn at atemperature at which practical mechanical strength is obtained,deterioration of transmission loss can be reduced, making it possible toprovide a dispersion-compensating optical fiber of the dispersionslope-compensating type, while also having low loss.

[0062] The dispersion-compensating optical fiber of the fifth embodimentof the present invention is able to compensate chromatic dispersion of asingle-mode optical fiber for transmission over a wide wavelength band,while also being able to enlarge Aeff and suppress non-linear effects.Accordingly, a hybrid transmission line can be provided that is suitablefor wavelength division multiplexing transmission and long-distance,high-speed transmission.

[0063] In additions since the absolute value of chromatic dispersion isnot reduced excessively even if Aeff is enlarged, chromatic dispersioncan be compensated of a single-mode optical fiber for transmission at acomparatively short used length.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064]FIG. 1 is a drawing showing an example of a segment corerefractive index distribution pattern representing the refractive indexdistribution of dispersion-compensating optical fibers of the first,second, third and fifth embodiments of the present invention.

[0065]FIG. 2 is a drawing showing an example of a so-called W-shapedrefractive index distribution pattern.

[0066]FIG. 3 is a drawing showing an example of the refractive indexdistribution pattern of a dispersion-compensating optical fiber of afourth embodiment of the present invention.

[0067]FIGS. 4A and 4B are graphs showing the relationship betweenbending loss and cutoff wavelength at a wavelength of 1.55 μm (FIG. 4A)or 1.63 μm (FIG. 4B) for the dispersion-compensating optical fiber of afourth embodiment of the present invention and a dispersion-compensatingoptical fiber provided with a segment core refractive index distributionpattern of the prior art.

[0068]FIGS. 5A and 5B are graphs showing the relationship betweenbending loss and Aeff at a wavelength of 1.55 μm (FIG. 5A) or 1.63 μm(FIG. 5B) for a dispersion-compensating optical fiber of a fourthembodiment of the present invention and a dispersion-compensatingoptical fiber provided with a segment core refractive index distributionpattern of the prior art.

[0069]FIGS. 6A through 6C are graphs comparing the dispersioncompensation characteristics of a segment core dispersion-compensatingoptical fiber of a fifth embodiment of the prevent invention and adispersion-compensating optical fiber provided with a W-shapedrefractive index distribution pattern for each wavelength range, withFIG. 6A showing the graph for a wavelength of 1.53-1.57 μm, FIG. 6B fora wavelength of 1.45-1.53 μm, and FIG. 6C for a wavelength of 1.53-1.63μm.

[0070]FIG. 7 is a graph showing an example of measurement of residualdispersion.

[0071]FIG. 8A is a graph showing the relationship between wavelength andchromatic dispersion of a dispersion-compensating optical fiber andsingle-mode optical fiber for transmission in Embodiment 5-1, while FIG.8B is a graph showing the relationship between wavelength and chromaticdispersion of a hybrid transmission line.

[0072]FIG. 9A is a graph showing the relationship between wavelength andchromatic dispersion of a dispersion-compensating optical fiber andsingle-mode optical fiber for transmission in Embodiment 5-2, while FIG.9B is a graph showing the relationship between wavelength and chromaticdispersion of a hybrid transmission line.

[0073]FIG. 10A is a graph showing the relationship between wavelengthand chromatic dispersion of a dispersion-compensating optical fiber andsingle-mode optical fiber for transmission in Comparative Example 5-1,while FIG. 10B is a graph showing the relationship between wavelengthand chromatic dispersion of a hybrid transmission line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] The following provides a detailed explanation of the presentinvention.

[0075] To begin with, an explanation is provided of a first embodiment.

First Embodiment

[0076] The dispersion-compensating optical fiber of a first embodimentof the present invention is able to compensate dispersion slope of a 1.3μm single-mode optical fiber by having a negative value for dispersionslope at a wavelength of 1.53-1.63 μm. Since the optimum value ofdispersion slope of a dispersion-compensating optical fiber variesaccording to the dispersion slope and used length of the 1.3 μmsingle-mode optical fiber to be compensated, although there are noparticular restrictions, it is preferably, for example, about −0.15 to−0.25 ps/nm²/km.

[0077] In addition, since a dispersion-compensating optical fibercompensates a single-mode optical fiber, it is necessary to performsingle-mode propagation at all times over the entire wavelength range of1.53-1.63 μm. Furthermore, in the actual state of long-distance use,single-mode propagation is possible even if the cutoff wavelengthaccording to the 2m-method recommended in CCITT-G.652 is longer than1.53 μm. Thus, in consideration of the used length and so forth, thecutoff wavelength must be a value that substantially guaranteessingle-mode propagation in the actual state of use.

[0078] Bending loss refers to the value under conditions in whichbending radius (2R) is 20 mm.

[0079] If bending loss is 30 dB/m or less over the entire wavelengthrange of 1.53-1.63 μm, this is desirable since this results inresistance to deterioration of optical characteristics even due tobending applied during fiber laying and so forth.

[0080] Effective area Aeff is defined with the following relationalexpression:${Aeff} = \frac{2\quad \pi \left\{ {\int_{0}^{\infty}{r{{E(r)}}^{2}\quad {r}}} \right\}^{2}}{\int_{0}^{\infty}{r{{E(r)}}^{4}\quad {r}}}$

[0081] r: radius, E(r): electric field strength at radius r

[0082] If Aeff is less than 20 μm² over the entire wavelength range of1.53-1.63 μm, non-linear effects cannot be adequately decreased.Furthermore, since it is preferable that Aeff be as large as possiblefrom the viewpoint of suppressing non-linear effects, although there areno particular restrictions on it, it is substantially 30 μm² or lessfrom the viewpoint of production ease and so forth.

[0083] In the dispersion-compensating optical fiber of a firstembodiment of the present invention, the absolute value of overallchromatic dispersion when a 1.3 μm single-mode optical fiber to becompensated is combined with a dispersion-compensating optical fiber is0.5 ps/nm or less over the entire wavelength range of 1.53-1.63 μm. As aresult, chromatic dispersion of a 1.3 μm single-mode optical fiber canbe compensated over the entire C and L-bands.

[0084] Here, the length and chromatic dispersion of a 1.3 μm single-modeoptical fiber used in optical communication systems and so forth variesaccording to the application and so forth.

[0085] Thus, the preferable used length and chromatic dispersion of adispersion-compensating optical fiber is suitably determined by the usedlength and chromatic dispersion of the 1.3 μm single-mode optical fiberto be compensated.

[0086] However, in the dispersion-compensating optical fiber of a firstembodiment of the present invention, the chromatic dispersion isrequired to be −50 ps/nm/km or less at a wavelength of 1.55 μm. In thecase the chromatic dispersion is larger than −50 ps/nm/km and approacheszero, there are disadvantages such as the used length of thedispersion-compensating optical fiber becoming long during compensationof a 1.3 μm single-mode optical fiber over the entire wavelength rangeof 1.53-1.63 μm.

[0087] Furthermore, in a dispersion-compensating optical fiber of theprior art, even if a dispersion-compensating optical fiber is attemptedto be designed to match the conditions of 1.3 μm single-mode opticalfibers in this manner, satisfactory characteristics were unable to beobtained particularly in the L-band. In contrast, thedispersion-compensating optical fiber of a first embodiment of thepresent invention is characterized by being able to adequatelycompensate chromatic dispersion of a 1.3 μm single-mode optical fiber inthe L-band as well.

[0088] For example, when using a 1.3 μm single-mode optical fiber havingchromatic dispersion of +21 ps/nm/km at 1.63 μm over a distance of 10km, if the chromatic dispersion of a dispersion-compensating opticalfiber at 1.63 μm is about −74 ps/nm/km, this dispersion-compensatingoptical fiber is able to compensate chromatic dispersion of this 1.3 μmsingle-mode optical fiber by using over a distance of 2.84 km.

[0089] As has been described above, although there are no particularrestrictions on the chromatic dispersion and used length of a 1.3 μmsingle-mode optical fiber since they vary according to the application,the chromatic dispersion of a 1.3 μm single-mode optical fiber at1.53-1.63 μm is normally about 16-22 ps/nm/km. In addition, the usedlength of the dispersion-compensating optical fiber of a firstembodiment of the present invention is set to about ⅙ to ⅓ the usedlength of a 1.3 μm single-mode optical fiber. If the used length is lessthan ⅙, there are cases in which chromatic dispersion is unable to beadequately compensated, and if the used length exceeds ⅓, there arecases in which transmission characteristics deteriorate.

[0090] A dispersion-compensating optical fiber that satisfies thesecharacteristic values is able to compensate chromatic dispersion anddispersion slope of a 1.3 μm single-mode optical fiber over a widewavelength band of 1.53-1.63 μm that combines the C-band and L-band, haslow bending loss and is resistant to the occurrence of non-lineareffects. It is preferable that these characteristics be satisfied in theL-band in particular.

[0091] A first condition for the dispersion-compensating optical fiberof a first embodiment of the present invention having the abovecharacteristics is possession of the refractive index distribution shownin FIG. 1.

[0092] This refractive index distribution pattern is provided with acore 4 and a cladding 5 provided around an outer periphery of the core4. This core 4 is provided with a three layers structure consisting ofthe central core portion 1 provided in the center, a intermediate coreportion 2 provided around an outer periphery of the central core portion1, and a ring core portion 3 provided around an outer periphery of theintermediate core portion 2.

[0093] The above cladding 5 is provided a substantially constantrefractive index.

[0094] In addition, the refractive indices of the central core portion 1and the ring core portion 3 are higher than that of the cladding 5,while the refractive index of the intermediate core portion 2 is lowerthan that of the cladding 5.

[0095] In this example, the intermediate core portion 2 is provided witha refractive index lower than the central core portion 1, and the ringcore portion 3 is provided with a refractive index that is higher thanthe intermediate core portion 2 and lower than the above central coreportion 1. The refractive index of the cladding 5 is lower than the ringcore portion 3 and higher than the above intermediate core portion 2.

[0096] In addition, Δ₁, Δ₂ and Δ₃ are the relative refractive indexdifferences of the central core portion 1, the intermediate core portion2 and the ring core portion 3, respectively, based on the refractiveindex of the cladding 5 (zero). In addition, a, b and c are the radii ofthe central core portion 1, the intermediate core portion 2 and the ringcore portion 3, respectively.

[0097] A refractive index distribution like that described below isreferred to as a segment core refractive index distribution pattern.

[0098] The above central core portion 1 and the ring core portion 3 arecomposed of, for example, GeO₂-doped SiO₂, and refractive index isadjusted by the amount of GeO₂ doped. The intermediate core portion 2 iscomposed of, for example, F-doped SiO₂, while the cladding 5 is composedof pure SiO₂ or SiO₂ which is containing at least one dopant selectedfrom F, Cl₂, or Ge.

[0099] In addition, 2 a is the outer diameter of the central coreportion 1 (a indicates ½ the outer diameter), 2 b is the inner diameterof the ring core portion 3 (b indicates ½ the inner diameter), w is thewidth of the ring core portion 3, Δ₂ is the relative refractive indexdifference between the cladding 5 and the intermediate core portion 2,Δ₃ is the relative refractive index difference between the cladding 5and the ring core portion 3, and Δ₁ is the relative refractive indexdifference between the cladding 5 and the central core portion 1.

[0100] The second condition is that, in the refractive indexdistribution pattern shown in FIG. 1, 2.5≦b/a≦5.0, 0.3≦w/a≦1.7, Δ₂ is−0.2 to −0.5%, Δ₃ is 0.1 to 1.3%, and Δ₁ is 1.5% or less, and preferably1.3% or less. These are the ranges of experimentally determined values.

[0101] Preferably, 2.7≦b/a≦3.5, 0.3≦w/a≦1.5, Δ₂ is −0.3 to −0.45%, Δ₃ is0.2 to 1% and Δ₁ is 0.8 to 1.5%.

[0102] More preferably, 2.8≦b/a≦3.1, 0.5≦w/a≦1, Δ₂ is −0.38 to −0.42%,Δ₃ is 0.4 to 1%, and Δ₁ is 1 to 1.2%.

[0103] If b/a is less than 2.5, chromatic dispersion cannot beadequately decreased, and if b/a is greater than 5.0, since opticalcharacteristics approach the characteristics of a matched cladding(single-peak) refractive index distribution pattern, Aeff cannot be,enlarged.

[0104] If w/a is less than 0.3, effects produced by the ring coreportion 3 diminish, and Aeff cannot be enlarged. If w/a is greater than1.7, the cutoff wavelength becomes longer thereby preventing single-modetransmission.

[0105] If Δ₂ is greater than Δ0.2%, dispersion slope cannot besufficiently reduced, and if it is less than −0.5%, transmission lossbecomes poor and FOM decreases.

[0106] If Δ₃ is less than 0.1%, effects produced by the ring coreportion 3 are eliminated and Aeff cannot be enlarged. In addition, if itis greater than 1.3%, the cutoff wavelength becomes longer preventingsingle-mode transmission.

[0107] If Δ₁ is greater than 1.5%, it is difficult to make Aeff greaterthan or equal to 20 μm².

[0108] Furthermore, the preferable values of Δ₂, Δ₃ and Δ₁ varyaccording to the values of b/a and w/a, and even if they are within therange of the above-mentioned experimentally determined values of Δ2, Δ3and Δ1, there is no assurance that an optical fiber having thecharacteristics of the dispersion-compensating optical fiber of a firstembodiment of the present invention will be obtained.

[0109] From this viewpoint, it is difficult to define the invention onlyby the values of structural parameters of a dispersion-compensatingoptical fiber in the first embodiment of the present invention, and wastherefore defined according to characteristic values.

[0110] It goes without saying, however, that such characteristic valuescannot be obtained with dispersion-compensating optical fibers known inthe prior art.

[0111] The dispersion-compensating optical fiber of a first embodimentof the present invention can be produced by a combination of ordinaryVAD and OVD, MCVD and so forth.

[0112] In the case of a segment core refractive index distributionpattern, the electric field strength distribution of optical power oftransmitted light is drawn out in the form of a long tail on the side ofthe cladding 5 due to the presence of the ring core portion 3, andduring production of the optical fiber base material, it is preferableto adopt a method in which a considerable portion of the suit that formsthe cladding is synthesized all at once simultaneous to the suit thatforms the central core.

[0113] Continuing, the following provides an explanation of a secondembodiment.

Second Embodiment

[0114] In a second embodiment of the present invention, the prescribedrange of wavelength width is selected according to the application from1.53-1.63 μm for the used wavelength band, examples of which include1.53-1.57 μm, 1.57-1.63 μm or the entire wavelength band of 1.53-1.63 μmand so forth.

[0115] Aeff is represented with the above expression.

[0116] The larger Aeff, the lower the non-linearity, thereby making thispreferable. In this second embodiment of the present invention, Aeff of30 μm² or more, and preferably 35 μm² or more, is obtained at the aboveused wavelength band. Although there are no particular restrictions onthe upper limit of Aeff, that within a range of 40 μm² or less can beproduced practically. If Aeff is less than 30 μm², suppressedly effectson non-linearity effects diminish, thereby making this disadvantageous.

[0117] In the second embodiment of the present invention, bending lossrefers to the value of a bending radius of 20 mm as previouslymentioned. It is preferable that bending loss be as low as possible. Inthe second embodiment of the present invention, bending loss of 40 dB/mor less, preferably 30 dB/m or less, and substantially within a range of0.1-10 dB/m is obtained at the above used wavelength band. If bendingloss exceeds 40 dB/m, loss increases due to the slightest bendingapplied when laying the optical fiber, thereby making thisdisadvantageous.

[0118] Chromatic dispersion at the above used wavelength band of thedispersion-compensating optical fiber of the second embodiment of thepresent invention is within the range of −40 to −10 ps/nm/km, andpreferably −18 to −25 ps/nm/km. If chromatic dispersion is greater than−10 ps/nm/km and approaches zero, it becomes difficult to compensate thechromatic dispersion of a single-mode optical fiber for transmission. Inthe case of that in which chromatic dispersion is less than −40ps/nm/km, Aeff ends up becoming small, substantially making productiondifficult.

[0119] In addition, when connected with a single-mode optical fiber fortransmission provided with positive chromatic dispersion and positivedispersion slope at a prescribed used length to compose a transmissionpath, the dispersion-compensating optical fiber of the second embodimentof the present invention is designed to be able to compensate thechromatic dispersion of the above single-mode optical fiber fortransmission so that the absolute value of chromatic dispersion of theentire transmission path is 4.0 ps/nm/km or less, and preferably 2.0ps/nm/km or less, while simultaneously being able to compensatedispersion slope of the above single-mode optical fiber for transmissionso that the absolute value of dispersion slope of the entiretransmission path is 0.03 ps/nm²/km or less, and preferably 0.01ps/nm²/km.

[0120] Although a typical example of single-mode optical fiber fortransmission that is the target of compensation is a 1.3 μm single-modeoptical fiber, there are no particular restrictions on the single-modeoptical fiber for transmission used provided it is provided with zerochromatic dispersion at wavelengths shorter than, for example, 1.53 μm,the lower limit of the used wavelength band of the second embodiment ofthe present invention.

[0121] The used length of a dispersion-compensating fiber is setaccording to the characteristics and so forth of the single-mode opticalfiber for transmission to be compensated as previously mentioned. In thesecond embodiment of the present invention, since thedispersion-compensating optical fiber itself is provided with low lossand low non-linear characteristics that enable it to be used as atransmission path, there is no risk of deterioration of transmissioncharacteristics over the entire transmission path even if the usedlength is comparatively long. Thus, although there are no particularrestrictions, the used length of the dispersion-compensating opticalfiber is 1.0 to 2.5 times the single-mode optical fiber fortransmission.

[0122] In addition, since the optimum dispersion slope varies accordingto the dispersion slope and used length of the single-mode optical fiberfor transmission to be compensated, although there are no particularrestrictions, it is preferably, for example, about −0.05 to −0.12ps/nm²/km.

[0123] In addition, since the dispersion-compensating optical fiber ofthe second embodiment of the present invention compensates single-modeoptical fibers, it is preferable that the cutoff wavelength have a valuethat guarantees single-mode propagation at the above used length.

[0124] Although the value measured according to the 2m-methodrecommended in CCITT-G.652 is normally used for the cutoff wavelength,single-mode propagation can still be performed even if the cutoffwavelength according to the 2m-method is longer than the used wavelengthin the state of use over long distances. Thus, cutoff wavelength adoptsa value that substantially guarantees single-mode propagation in theactual state of use in consideration of the used length.

[0125] In the dispersion-compensating optical fiber of the secondembodiment of the present invention, being provided with the refractiveindex distribution shown in FIG. 1 is a condition for obtaining thesecharacteristics.

[0126] In this refractive index distribution, for example, the centralcore portion 1 and the ring core portion 3 are formed fromgermanium-doped quartz glass, the intermediate core portion 2 is formedfrom fluorine-doped quartz glass, and the cladding 5 is formed from purequartz glass or quartz glass which is containing at least one dopantselected from fluorine, chlorine or germanium.

[0127] Furthermore, the cladding 5 preferably has a refractive indexthat is equal to or less than the value of the refractive index of purequartz glass. The reason for this is because the stress remaining in thecentral core portion 1 and so forth after drawing can be reduced bydecreasing the difference in softening temperature between core 4 andthe cladding 5, thereby allowing the obtaining of an optical fiber withlow loss.

[0128] In this refractive index distribution, when radii and relativerefractive index differences based on the cladding 5 of the central coreportion 1, the intermediate core portion 2 and the ring core portion 3are expressed as (a,Δ₁), (b,Δ₂) and (c,Δ₃), respectively, it ispreferable that 2.0≦b/a≦3.0, 2.5≦c/a≦4.0, Δ₁ is 0.6 to 0.9%, Δ₂ is −0.30to −0.50%, and Δ₃ 0.4 to 0.9%.

[0129] If b/a is less than 2.0, The effect of the intermediate coreportion 2 diminishes and Aeff decreases, while if b/a exceeds 3.0,bending loss increases, thereby preventing the optical fiber from beingused. If c/a is less than 2.5, the effect of the ring core portion 3diminishes and Aeff decreases, while if c/a exceeds 4.0, bending lossdecreases, thereby preventing the optical fiber from being used.

[0130] In addition, if Δ₁ is less than 0.6%, bending loss increases,while if it exceeds 0.9%, Aeff decreases. In addition, if Δ₂ is greaterthan −0.30% (and approaches the refractive index of the cladding 5),Aeff decreases, while if it is less than −0.50%, bending loss increasesthereby preventing the optical fiber from being used. In addition, if Δ₃is less than 0.4%, the effect of the ring core portion 3 diminishes andAeff decreases, while if it exceeds 0.9%, bending loss increases,thereby preventing the optical fiber from being used.

[0131] Even if the dispersion-compensating optical fiber of the secondembodiment of the present invention has the refractive indexdistribution shown in FIG. 1 and satisfies the above range of values foreach structural parameter, there is no guarantee that adispersion-compensating optical fiber will be obtained that is providedwith the above characteristics. Namely, only by selecting suitablevalues by trial and error so as to be able to realize the abovecharacteristics from the five ranges of values for b/a, c/a, Δ₁, A₂ andA₃ is it only possible to obtain a dispersion-compensating optical fiberof the second embodiment of the present invention.

[0132] Thus, since it is difficult to define the present invention onlyby the ranges of values pertaining to structural parameters of adispersion-compensating optical fiber in the dispersion-compensatingoptical fiber of the second embodiment of the present invention, it wastherefore defined according to characteristic values. Furthermore, itgoes without saying, however, that such characteristic values cannot beobtained with dispersion-compensating optical fibers known in the priorart.

[0133] The dispersion-compensating optical fiber of a second embodimentof the present invention can be produced by a combination of ordinaryVAD and OVD, MCVD, PCVD and so forth.

[0134] The following provides an explanation of a third embodiment ofthe present invention.

Third Embodiment

[0135] The dispersion-compensating optical fiber of a third embodimentof the present invention is provided with a refractive indexdistribution pattern similar to that shown in FIG. 1.

[0136] In the dispersion-compensating optical fiber of the thirdembodiment of the present invention, a shown in FIG. 1 is set at 2-3 μm.If a is less than 2 μm, bending loss increases, while if a exceeds 3 μm,Aeff decreases and cutoff wavelength becomes longer, thereby preventingthe desired characteristics from being obtained.

[0137] In addition, when radii (½ outer diameters) and relativerefractive index differences based on the cladding 5 of the central coreportion 1, the intermediate core portion 2 and the ring core portion 3are expressed as (a,Δ₁), (b,Δ₂) and (c,Δ₃), respectively, Δ₁ ispreferably 0.9-1.5%. If less than 0.9%, there are cases in which thedesired chromatic dispersion and dispersion slope are not obtained. Ifgreater than 1.5%, the amount of dopant doped to the central coreportion 1 increases resulting in increased transmission loss. Inaddition, there is increased susceptibility to the occurrence ofnon-linear effects due to a decrease in Aeff.

[0138] Δ₂ is −0.30 to −0.45%. If less than −0.45%, transmission lossdeteriorates easily, and if greater than −0.30%, dispersion slopecompensation rate deteriorates.

[0139] Δ₃ is 0.2-1.2%. If less than 0.2%, Aeff decreases resulting inincreased susceptibility to the occurrence of non-linear effects, whileif greater than 1. 2%, cutoff wavelength becomes longer preventing theobtaining of the desired characteristics.

[0140] Moreover, b/a is 2.0-3.5. If less than 2.0, dispersion slopecompensation rate deteriorates, while if greater than 3.5, bending lossdeteriorates.

[0141] In addition, c/a is 3.0-5.0. If less than 3.0, Aeff decreasesresulting in increased susceptibility to the occurrence of non-lineareffects, while if greater than 5.0, cutoff wavelength becomes longerpreventing the obtaining of the desired characteristics.

[0142] Furthermore, although there are no particular restrictions on theouter diameter of the cladding 5 (outer diameter of thedispersion-compensating optical fiber), it is normally about 125 μm.

[0143] In the third embodiment of the present invention, even if Δ₁ hasa comparatively small range in this manner, the preferablecharacteristics indicated below can be realized by suitably combining aplurality of other structural parameters.

[0144] Furthermore, even if the ranges of all of these values aresatisfied, there is no guarantee that a dispersion-compensating opticalfiber provided with the following characteristics can be obtained.Namely, a combination of a plurality of suitable structural parametersallowing the obtaining of the following characteristics can only beobtained by trial and error.

[0145] Thus, since it is difficult to define the invention only byrefractive index distribution pattern and the ranges of values ofstructural parameters in the dispersion-compensating optical fiber ofthe third embodiment of the present invention, it is defined by thefollowing characteristic values in addition to these constructions.

[0146] In this manner, a dispersion-compensating optical fiber having asmall value for Δ₁, superior chromatic dispersion and dispersion slopecompensating effects and enlarged Aeff for suppressing non-lineareffects was unable to be obtained in the prior art.

[0147] The used wavelength band in the third embodiment of the presentinvention refers to a wavelength band selected from 1.53-1.63 μm. Therange of wavelength width of the used wavelength band can be suitablyselected as necessary, and may substantially be a single wavelength.Furthermore, a comparatively wide wavelength band is selected forwavelength division multiplexing transmission and so forth, examples ofwavelength bands that can be selected including 1.53-1.57 μm (so-calledC-band) and 1.55-1.63 μm (so-called L-band).

[0148] The dispersion-compensating optical fiber of the third embodimentof the present invention has an Aeff of 20 μm² or more, and preferably26 μm² or more, at the selected used wavelength. Since Δ₁ is small, thiskind of large Aeff can be realized. Although there are no particularrestrictions on the upper limit of Aeff, an optical fiber having an Aeffof 30 μm² or less can be substantially produced. If Aeff is less than 20μm², non-linear effects are unable to be suppressed thereby making thisdisadvantageous.

[0149] Furthermore, Aeff is defined by the previously mentionedexpression.

[0150] In addition, although bending loss is preferably as small aspossible, the dispersion-compensating optical fiber of the thirdembodiment of the present invention has bending loss of 40 dB/m or less,and preferably 20 dB/m or less, at the selected used wavelengthmentioned above. If bending loss is 40 dB/m or less, there is littledeterioration of transmission loss caused by even slight stress appliedwhen laying the optical fiber and so forth, thereby allowing theobtaining of stable characteristics.

[0151] Furthermore, bending loss is the value obtained under conditionsof the bending radius (2R) being 20 mm as previously mentioned.

[0152] Moreover, the dispersion-compensating optical fiber of the thirdembodiment of the present invention has chromatic dispersion of −65 to−45 ps/nm/km at the selected used wavelength mentioned above.

[0153] If within this range, it is possible to compensate chromaticdispersion of single-mode optical fibers provided with comparativelylarge positive chromatic dispersion in this used wavelength band and azero dispersion wavelength that is shorter than above used wavelengthband typically represented by 1.3 μm single-mode optical fibers by adispersion-compensating optical fiber of a comparatively short length.

[0154] In addition, it is necessary that the dispersion-compensatingoptical fiber of the third embodiment of the present invention be asingle-mode optical fiber. Namely, it must be provided with a cutoffwavelength that is able to maintain single-mode propagation in the stateof actual use. Although the value measured by the so-called 2m-method isnormally used for the cutoff wavelength, in the case of actual use overlong distances, single-mode propagation can be performed even if thecutoff wavelength as determined by the 2m-method is longer than theshortest wavelength of the used wavelength band.

[0155] Thus, a suitable upper limit is set for the cutoff wavelengthaccording to the used wavelength band and used length, and adispersion-compensating optical fiber is designed so as to realize avalue that does not exceed this upper limit.

[0156] In addition, the dispersion slope of the dispersion-compensatingoptical fiber of the third embodiment of the present invention is suchthat the compensation rate of dispersion slope when using adispersion-compensating optical fiber of a length that is able tocompensate the chromatic dispersion of a single-mode optical fibercompensated by this dispersion-compensating optical fiber to zero is80-120%. If outside of this range, compensation of dispersion slopebecomes inadequate, thereby possibility causing difficulties duringwavelength division multiplexing transmission and so forth.

[0157] The compensation rate of dispersion slope is determined in themanner described below.

[0158] The absolute values of chromatic dispersion and dispersion slopeper unit length of a single-mode optical fiber in the used wavelengthband are respectively taken to be d1 (ps/nm/km) and s1 (ps/nm²/km),while the absolute values of chromatic dispersion and dispersion slopeper unit length of a dispersion-compensating optical fiber arerespectively taken to be d2 (ps/nm/km) and s2 (ps/nm²/km).

[0159] The chromatic dispersion and dispersion slope of the abovesingle-mode optical fiber are normally positive values. The chromaticdispersion and dispersion slope of the dispersion-compensating opticalfiber of the third embodiment of the present invention are normallynegative values.

[0160] To begin with, the length of a dispersion-compensating opticalfiber able to compensate the dispersion wavelength of a single-modeoptical fiber of a unit length to zero is expressed as d1/d2.

[0161] The dispersion slope of a dispersion-compensating optical fiberat this length is expressed as d1/d2×s2. The compensation rate ofdispersion slope of a single-mode optical fiber per unit length by adispersion-compensating optical fiber of this length is expressed as(d1/d2×s2)/s1×100.

[0162] In this manner, since the compensation rate of dispersion slopevaries according to the chromatic dispersion and dispersion slope of thesingle-mode optical fiber to be compensated in the used wavelength band,and the chromatic dispersion and dispersion slope of thedispersion-compensating optical fiber itself, it is necessary to designthe dispersion-compensating optical fiber to match the target usedwavelength band and single-mode optical fiber.

[0163] In the dispersion-compensating optical fiber of the thirdembodiment of the present invention, the dispersion slope of asingle-mode optical fiber provided with a zero dispersion wavelengthshorter than the above used wavelength band, as is represented by 1.3 μmsingle-mode optical fibers, can be adequately compensated within therange of the compensation rate of this dispersion slope by a suitablecombination of structural parameters selected from the above range ofvalues.

[0164] For example, that over a range of −0.13 to −0.27 ps/nm²/km can bearbitrarily set as the negative dispersion slope of adispersion-compensating optical fiber.

[0165] In addition, in the refractive index distribution pattern shownin FIG. 1, it is preferable that dopant be doped to each layer. Inparticular, although there are many cases in which the cladding 5 iscomposed of pure quartz glass as previously described, in order toobtain a dispersion-compensating optical fiber having low loss byreducing the difference in viscosity with core 4 during drawing toincrease the drawing speed, it is effective to add dopant to thecladding 5.

[0166] Namely, the central core portion 1 and the ring core portion 3are preferably formed from germanium-doped quartz glass, theintermediate core portion 2 is preferably formed from fluorine-dopedquartz glass, and the cladding 5 is formed from quartz glass which iscontaining at least one dopant selected from fluorine, chlorine, andgermanium, preferably quartz glass doped with a small amount offluorine.

[0167] Furthermore, when doping the cladding 5 with fluorine, if theamount doped is adjusted so that, for example, the relative refractiveindex difference based on pure quartz is about −0.1 to −0.4%, adequateeffects can be obtained.

[0168] This dispersion-compensating optical fiber can be produced usingknown methods such as VAD, MCVD or PCVD and so forth.

[0169] In this manner, since Δ₁ of the dispersion-compensating opticalfiber of the third embodiment of the present invention is small and thedifference in viscosity between core 4 and the cladding 5 during drawingis small, the mechanical strength of this dispersion-compensatingoptical fiber does not tend to lower even if the drawing speed isincreased beyond that of dispersion-compensating optical fibers of theprior art. Consequently, a dispersion-compensating optical fiber havinglow loss can be obtained by drawing at higher speeds than in the priorart. In addition, the occurrence of non-linear effects can be suppressedas a result of enlarging Aeff.

[0170] In addition, the dispersion-compensating optical fiber of thethird embodiment of the present invention allows the construction of ahybrid transmission line suitable for wavelength division multiplexingtransmission, long-distance transmission and so forth by combining witha single-mode optical fiber.

[0171] The length and so forth of the dispersion-compensating opticalfiber and single-mode optical fiber used in a hybrid transmission lineare set to suitable values according to the characteristics and designconditions of each fiber.

[0172] Furthermore, normally a single-mode optical fiber is arranged ina former stage, and a dispersion-compensating optical fiber is arrangedin a latter stage so as to compensate chromatic dispersion anddispersion slope that have accumulated due to propagation of thissingle-mode optical fiber.

[0173] Although the combined single-mode optical fiber may be a 1.3 μmsingle-mode optical fiber as was previously mentioned, there are noparticular restrictions on this single-mode optical fiber provided ithas a zero dispersion wavelength that is shorter than the usedwavelength band, and is provided with positive chromatic dispersion andpositive dispersion slope in the used wavelength band.

[0174] For example, the use of a single-mode optical fiber like thatindicated below is preferable.

[0175] This single-mode optical fiber has the so-called W-shapedrefractive index distribution pattern shown in FIG. 2. This refractiveindex distribution pattern is provided with a core 14 and a cladding 15provided around an outer periphery of the core 14. The core 14 isprovided with a central core portion 11, which is provided with arefractive index higher than the above cladding 15, and intermediatecore portion 12, which is provided around an outer periphery of coreportion 11 and is provided with a refractive index lower than thiscladding 15. In the drawing, a₁ is the radius of the central coreportion 11, b₁ is the radius of intermediate core portion 12 (radius ofcore 14), and Δ₁₁ and Δ₁₂ are the relative refractive index differencewith the central core portion 11 and the relative refractive indexdifference with intermediate core portion 12, respectively, when basedon the refractive index of cladding 15 (zero). Namely, Δ₁₁ is a positivevalue and Δ₁₂ is a negative value.

[0176] This single-mode optical fiber has Aeff of 120 μm² or more in theused wavelength band, and has a cutoff wavelength that substantiallyallows single-mode propagation.

[0177] If Aeff is 120 μm² or more, since there is less susceptibility tothe occurrence of non-linear effects, the resulting single-mode opticalfiber has low loss, and characteristics are obtained that areparticularly suitable for long-distance, wavelength divisionmultiplexing transmission and so forth.

[0178] In addition, the cutoff wavelength is a condition for being asingle-mode optical fiber.

[0179] In this single-mode optical fiber as well, a single-mode opticalfiber can be obtained provided with the above conditions for Aeff andcutoff wavelength by selecting and combining suitable values for a₁, b₁,Δ₁₁ and Δ₁₂.

[0180] b₁/a₁ is selected from a range of, for example, 3.0-5.0. If lessthan 3.0, since the optical electromagnetic field is able to easilyextend beyond intermediate core portion 12 and reach cladding 15,bending loss tends to increase. In addition, if greater than 5.0, theeffect of providing intermediate core portion 12 diminishes and trappingof the optical electromagnetic filed in the core becomes excessivelystrong, which tends to decrease the effect of enlarging Aeff.

[0181] In addition, the cutoff wavelength can be shifted towards alonger wavelength by increasing the value of a₁. As was mentioned above,since cutoff wavelength is set according to the used length andwavelength band of the optical fiber, it is generally not possible toindicate the range of values of a₁. However, a₁ is normally selectedfrom a range of 5-20 μm.

[0182] The outer diameter of cladding 15 is normally about 125 μm.

[0183] In addition, Δ₁₁ is 0.3% or less, and preferably 0.26% or less,while Δ₁₂ is preferably −0.05 to −0.15%.

[0184] If Δ₁₁ exceeds 0.3%, it becomes difficult to enlarge Aeff. Inaddition, if Δ₁₂ is greater than −0.05% (when the absolute value of Δ₁₂becomes smaller), bending loss increases. If Δ₁₂ is smaller than −0.15%(when the absolute value of Δ₁₂ becomes larger), Aeff tends to decrease.

[0185] Moreover, chromatic dispersion is preferably +19 to +22 ps/nm/km,dispersion slope preferably +0.065 ps/nm²/km or less, and bending losspreferably 10 dB/m or less.

[0186] Furthermore, the central core portion 11 is preferably composedof germanium-doped quartz glass, intermediate core portion 12 ispreferably composed of fluorine-doped quartz glass, and cladding 15 iscomposed of pure quartz glass or quartz glass which is containing atleast one dopant selected from fluorine, chlorine, and germanium,preferably fluorine-doped quartz glass.

[0187] This single-mode optical fiber can be produced by commonproduction methods in the same manner as the above-mentioneddispersion-compensating optical fiber.

[0188] Combining this single-mode optical fiber with thedispersion-compensating optical fiber of the third embodiment of thepresent invention makes it possible to construct a hybrid transmissionline having low non-linearity, low loss, little chromatic dispersion andlow dispersion slope.

[0189] The following provides an explanation of a fourth embodiment ofthe present invention.

Fourth Embodiment

[0190] The dispersion-compensating optical fiber as claimed in a fourthembodiment of the present invention is provided with a refractive indexdistribution pattern like that shown in FIG. 3.

[0191] This refractive index distribution pattern is provided with acore 20 and a cladding 25 provided around an outer periphery of the core20. Said core 20 is provided with a central core portion 21 having ahigher refractive index than the above cladding 25, a intermediate coreportion 22 provided around an outer periphery of said central coreportion 21 and having a refractive index lower than the above cladding25, a ring core portion 23 provided around an outer periphery of saidintermediate portion 22 and having a refractive index higher than theabove cladding 25, and a side ring core portion 24 provided around anouter periphery of said ring core portion 23 and having a refractiveindex lower than the above cladding 25.

[0192] In addition, in this example, the refractive index of the ringcore portion 23 is lower than that of the central core portion 21, andthe refractive index of the side ring core portion 24 is higher thanthat of the intermediate core portion 22.

[0193] Furthermore, in the drawing, a, b, c and d represent the radii (½of outer diameters) of the central core portion 21, the intermediatecore portion 22, the ring core portion 23 and the side ring core portion24, respectively, and Δ₂₁, Δ₂₂, Δ₂₃ and Δ₂₄ represent the relativerefractive index differences based on the cladding 25 of the centralcore portion 21, the intermediate core portion 22, the ring core portion23 and the side ring core portion 24, respectively.

[0194] In this example, the central core portion 21 and the ring coreportion 23 are formed from quartz glass containing a dopant thatprovides the effect of increasing the refractive index, while theintermediate core portion 22 and the side ring core portion 24 areformed from quartz glass containing a dopant that provides the effect oflowering the refractive index. Germanium (Ge) is a typical example of adopant that has the effect of increasing refractive index, and germaniumis doped in the form of GeO₂. In addition, fluorine (F) is a typicalexample of a dopant that has the effect of lowering refractive index.

[0195] Furthermore, at least one type or two or more types of dopantsselected from the group consisting of germanium, aluminum (Al),phosphorous (P) and fluorine are used for the dopant doped to thecentral core portion 21, the intermediate core portion 22, the ring coreportion 23, the side ring core portion 24 and the cladding 25, and thetype(s) of dopant and amount doped are suitably selected according tothe desired refractive index.

[0196] In this dispersion-compensating optical fiber, the cladding 25 isformed from quartz glass containing dopant. This dopant is provided withthe effect of lowering refractive index, and a typical example of such adopant is fluorine as mentioned above. As a result, the softeningtemperature of the cladding 25 becomes lower than pure quartz glass. Inother words, the base refractive index of Δ₂₁, Δ₂₂, Δ₂₃ and Δ₂₄ (zero)is lower than the refractive index of pure quartz glass.

[0197] Thus, the amount of dopant doped is less in the central coreportion 21 and the ring core portion 23 provided with higher refractiveindices than the cladding 25. The decreases in softening temperature andhardening temperature due to addition of dopant can therefore be reducedin the central core portion 21 and the ring core portion 23 more than inthe case of being based on the cladding 25 composed of pure quartzglass. In addition, the difference in viscosity of each layer in theproximity of 1900° C. can also be reduced.

[0198] Although the differences in softening temperature and hardeningtemperature between the cladding 25, comprised of pure quartz glass, andthe central core portion 21, containing a large amount of dopant, inparticular presented a problem in the prior art, in the fourthembodiment of the present invention, since the softening and hardeningtemperatures of the cladding 25 are lowered while the softening andhardening temperatures of the central core portion 21 are raised, thedifference between their softening and hardening temperatures issmaller, and the viscosity difference at the drawing temperature is alsosmaller. As a result, the amount of stress remaining in the portion tothe inside of the cladding 25, and particularly in the central coreportion 21, after drawing can be reduced even if drawn at the drawingtemperature guaranteed by the mechanical strength of thedispersion-compensating fiber, thereby making it possible to reducedeterioration of transmission loss caused by this.

[0199] The relative refractive index difference of the central coreportion 21 (Δ₂₁) based on the cladding 25 (zero) is set at 0.90-1.30%,and preferably 0.90-1.00%. If it is less than 0.90% and approaches zero,the amount of dopant doped decreases thereby preventing the softeningand hardening temperature from being raised sufficiently. If greaterthan 1.30%, the amount of dopant doped increases, resulting in apossible increase in transmission loss.

[0200] The relative refractive index difference of the side ring coreportion 24 (Δ₂₄) based on the cladding 25 (zero) is set at −0.50 to0.00%, and preferably −0.25 to −0.02%. If it is less than −0.50% and theamount of dopant doped increases, the amount of dopant doped of Δ₂₂increases which may result in deterioration of transmission loss. If itis greater than 0.00%, the amount of dopant doped decreases, therebypreventing the softening and hardening temperature from being loweredsufficiently.

[0201] The dispersion-compensating optical fiber of the fourthembodiment of the present invention is obtained by producing acylindrical fiber base material in which dopant is doped to each layerby a known method such as VAD, MCVD or PCVD, arranging so that thedirection of length of this fiber base material is in the verticaldirection, and drawing the lower end of this fiber base material byheating.

[0202] In general, the outer diameter of the fiber base material is30-80 mm, and the outer diameter of the dispersion-compensating opticalfiber is 80-125 μm. In addition, the heating temperature during drawingof the dispersion-compensating optical fiber of the fourth embodiment ofthe present invention is 1800-2100° C., and the drawing speed is 100-300m/min. In addition, the drawing tension at this time is to be 100-200 g.Practical mechanical strength is obtained under these conditions.

[0203] The entire portion from the central core portion 21 to thecladding 25 of the dispersion-compensating optical fiber of the fourthembodiment of the present invention is composed of quartz glasscontaining dopant, and since the softening and hardening temperaturesare low, the heating temperature of the fiber base material can be lowerthan that of the prior art provided with the cladding 25 composed ofpure quartz glass.

[0204] Furthermore, the refractive index pattern of an actualdispersion-compensating optical fiber is in the shape of a gentle curve,and the boundaries between each component portion are indistinct.Accordingly, as will be described below, it is preferable to producethis optical fiber by first setting the values of Δ₂₂ and otherstructural parameters, and then make fine adjustments while monitoringoptical characteristics during actual production.

[0205] In this manner, in the fourth embodiment of the presentinvention, by forming the cladding 25 from quartz glass containingdopant, the differences in softening temperatures and hardeningtemperatures between the central core portion 21 and the cladding 25 canbe decreased, and the difference in viscosity between the central coreportion 21 and the cladding 25 during drawing can be made smaller.

[0206] As a result, the amount of stress remaining in core portion 21and so forth after drawing can be reduced, thereby making it possible toreduce deterioration of transmission loss even if drawn at a temperatureat which practical mechanical strength is obtained.

[0207] By suitably setting Δ₂₁, Δ₂₂, Δ₂₃, Δ₂₄, b/a, c/a and d/a in thedispersion-compensating optical fiber provided with the refractive indexdistribution pattern of the fourth embodiment of the present invention,desirable characteristics can be realized for chromatic dispersion,dispersion slope, bending loss and so forth.

[0208] This type of refractive index distribution pattern is preferablefrom the viewpoint of suppressing non-linear effects as a result ofbeing able to enlarge Aeff to 20 μm² or more, preferably 25 μm² or moreas previously mentioned. In addition, since bending loss can also bedecreased at longer wavelengths, this is also preferable from theviewpoint of using in the L-band (1.57-1.63 μm).

[0209] The used wavelength band of the dispersion-compensating opticalfiber of the fourth embodiment of the present invention is selected fromthe range of 1.45-1.63 μm. For example, the used wavelength band issuitably selected from the range of 1.45-1.57 μm, 1.57-1.63 μm, orcombining both and selected from the range of 1.45-1.63 μm, according tothe amplification wavelength band of an Er-doped optical fiberamplifier.

[0210] In addition, chromatic dispersion in the used wavelength band ofthe dispersion-compensating optical fiber of the fourth embodiment ofthe present invention is set at −70 to −45 ps/nm/km. In the case it isgreater than −45 ps/nm/km and approaches zero, the used length becomeslonger which is disadvantageous. If chromatic dispersion is less than−70 ps/nm/km, characteristics deteriorate easily resulting in productionbeing difficult.

[0211] The object of the dispersion-compensating optical fiber of thefourth embodiment of the present invention is to compensate chromaticdispersion and dispersion slope of a single-mode optical fiber fortransmission having positive chromatic dispersion in the above usedwavelength band in the manner of a 1.3 μm single-mode optical fiber.

[0212] Accordingly, single-mode optical fibers for transmission targetedfor compensation by the dispersion-compensating optical fiber of thefourth embodiment of the present invention are not only 1.3 μmsingle-mode optical fibers, but also include single-mode optical fibershaving a zero dispersion wavelength shorter than the used wavelengthband in which chromatic dispersion increases at wavelengths longer thanthis zero dispersion wavelength. This type of single-mode optical fibernormally has a positive dispersion slope.

[0213] It is preferable that the dispersion slope of thedispersion-compensating optical fiber of the fourth embodiment of thepresent invention be such that a compensation rate of the dispersionslope using a dispersion-compensating optical fiber of a length thatreduces the chromatic dispersion of the combined single-mode opticalfiber for transmission to zero is 80-120% during compensation of thissingle-mode optical fiber. If within this range, dispersion slope can beadequately compensated, allowing the obtaining of satisfactorywavelength division multiplexing transmission characteristics.

[0214] The above compensation rate of the dispersion slope defined asRDS(DCF)/RDS (single-mode optical fiber)×100, when the value obtained bydividing the dispersion slope of the single-mode optical fiber bychromatic dispersion of the single-mode optical is taken to be RDS(single-mode optical fiber), and the value obtained by dividing thedispersion slope of the dispersion-compensating optical fiber bychromatic dispersion of the dispersion-compensating optical fiber istaken to be RDS (DCF).

[0215] As has been described above, since the compensation rate ofdispersion slope varies according to the chromatic dispersion anddispersion slope of the single-mode optical fiber for transmissiontargeted for compensation in the used wavelength band, and the chromaticdispersion and dispersion slope of the dispersion-compensating opticalfiber itself, it is necessary to design a dispersion-compensatingoptical fiber according to the target used wavelength band and thesingle-mode optical fiber for transmission.

[0216] In addition, bending loss refers to the value under the conditionof a bending radius (2R) of 20 mm as previously mentioned in the usedwavelength band. In the dispersion-compensating optical fiber of thefourth embodiment of the present invention, the bending loss in the usedwavelength band (preferably wavelength 1.63 μm band) is preferably 50dB/m or less. If it exceeds 50 dB/m, there are cases in whichtransmission characteristics deteriorate due to even the slightestbending applied when laying the optical fiber and so forth.

[0217] In the dispersion-compensating optical fiber of a fourthembodiment of the present invention shown in FIG. 3, the value of Δ₂₁ isdetermined relatively from the relationship with other structuralparameters and so forth, and has a wider range than the range of thevalue of Δ₁₁ of a so-called W-shaped refractive index distributionpattern shown in FIG. 2, and the range of the value of Δ₁ of a segmentcore refractive index distribution pattern shown in FIG. 1.

[0218]FIGS. 4A and 4B are graphs showing the relationship betweenbending loss and cutoff wavelength at a wavelength of 1.55 μm or 1.63 μmwhen targeting Aeff=25-28 μm² for a dispersion-compensating opticalfiber provided with a segment core refractive index distribution patternof the prior art and the dispersion-compensating optical fiber of thefourth embodiment of the present invention.

[0219]FIGS. 5A and 5B are graphs showing the relationship betweenbending loss and Aeff for a dispersion-compensating optical fiberprovided with a segment core refractive index distribution pattern ofthe prior art, and the dispersion-compensating optical fiber of thefourth embodiment of the present invention.

[0220] According to FIGS. 4A and 4B and FIGS. 5A and 5B, when comparedwith a dispersion-compensating optical fiber of the prior art, it can beunderstood that the dispersion-compensating optical fiber of the fourthembodiment of the present invention is able to lower bending loss eitherin the case of having the same cutoff wavelength or the case of havingthe same Aeff.

[0221] Furthermore, the points shown in these graphs assume asingle-mode optical fiber for transmission in which the RDS value(single-mode optical fiber), determined by dividing the chromaticdispersion slope of the single-mode optical fiber for transmission byits chromatic dispersion, is about 0.003.

[0222] In addition, it is also assumed that wavelength compensation isperformed using a ratio of 3:1 for the ratio of the length of thesingle-mode optical fiber for transmission to the length of thedispersion-compensating optical fiber, and that the single-mode opticalfiber for transmission is able to adequately withstand lateral pressure.

[0223] If the side ring core portion of this dispersion-compensatingoptical fiber for transmission is too large, the cutoff wavelengthbecomes longer and the dispersion-compensating optical fiber fortransmission becomes susceptible to lateral pressure. In order toprevent this, Δ₂₄ is −0.50 to 0.00%, and preferably −0.25 to −0.02%,while the normalized frequency (V4) is −15.0 to 0.0, and preferably−10.0 to −1.0. Here, the normalized frequency refers to the frequencyresulting from normalizing the optical frequency using the structuralparameters of an optical waveguide.

[0224] In addition, although it is effective to enlarge Aeff in order tosuppress non-linear effects, the value of Δ₂₁ must be decreased in orderto accomplish this. However, if the value of Δ₂₁ is too small, thedispersion-compensating optical fiber for transmission becomessusceptible to lateral pressure. Thus, the value of Δ₂₁ is preferably0.90-1.00%.

[0225] An optical fiber transmission path combining thedispersion-compensating optical fiber of the fourth embodiment of thepresent invention with an optical fiber in which Aeff is 70 μm² or moreat a used wavelength selected from 1.45-1.63 μm, and has a cutoffwavelength that allows single-mode propagation, has superior effectsthat suppress increases in transmission loss.

[0226] Namely, this is because, in the dispersion-compensating opticalfiber of the fourth embodiment of the present invention, non-lineareffects can be suppressed in an optical fiber in which dispersion slopeapproaches zero that is selected in the manner described above.

[0227] The following provides an explanation of a fifth embodiment ofthe present invention.

Fifth Embodiment

[0228] The dispersion-compensating optical fiber of a fifth embodimentof the present invention is provided with the refractive indexdistribution pattern shown in FIG. 1.

[0229] This dispersion-compensating optical fiber is provided with core4 and the cladding 5 provided around an outer periphery of the core 4.This core 4 is provided with a three-layer structure comprised of thecentral core portion 1 provided in its center, the intermediate coreportion 2 provided around an outer periphery of said the central coreportion 1, and the ring core portion 3 provided around an outerperiphery of said the intermediate core portion 2.

[0230] The above the cladding 5 is provided a substantially constantrefractive index.

[0231] In addition, the refractive indices of the central core portion 1and the ring core portion 3 are higher than that of the cladding 5,while the refractive index of intermediate core portion is lower thanthat of the cladding 5.

[0232] The central core portion 1 and the ring core portion 3 arecomposed of, for example, germanium-doped quartz glass, the intermediatecore portion 2 is composed of, for example, pure quartz glass orfluorine-doped quartz glass, and the cladding 5 is composed of, forexample, pure quartz glass or quartz glass which is containing at leastone dopant selected from fluorine, chlorine and germanium.

[0233] By using this type of refractive index distribution pattern andsuitably setting the ratios of differences in specific refractivity andradius of each layer, a dispersion-compensating optical fiber can beobtained that has little fluctuation in chromatic dispersion at longwavelengths in particular and can be used over a broad wavelength bandsuch as from the S-band to the C-band or from the C-band to the L-band.Aeff can also be simultaneously enlarged to suppress non-linear effects.

[0234] This dispersion-compensating optical fiber can be produced bydrawing from a fiber base material obtained by a known method such asVAD, MCVD or PCVD. In addition, if substantially provided with therefractive index distribution pattern shown in FIG. 1, there is no needfor a refractive index diffraction pattern completely in the shape ofsteps having a well-defined border between each layer, but rather may bein the form of a pattern that changes gradually.

[0235] The used wavelength band of the dispersion-compensating opticalfiber of the fifth embodiment of the present invention is selected acontinuous range provided with a wavelength width of 0.06 μm or more,and preferably 0.10 μm or more from a used wavelength of 1.45-1.63 μm.In the fifth embodiment of the present invention, the preferablecharacteristics described below can be realized in a broad wavelengthband as described above.

[0236] In addition, since the larger the absolute value of chromaticdispersion of the dispersion-compensating optical fiber, the greater thecompensation of chromatic dispersion at a short used wavelength relativeto the length of the single-mode optical fiber for transmission, this ispreferable from the viewpoint of cost, transmission loss and so forth.

[0237] However, since Aeff becomes smaller the larger the absolute valueof chromatic dispersion, this is disadvantageous from the viewpoint ofsuppressing non-linear effects.

[0238] Consequently, it is preferable to set the chromatic dispersion ofthe dispersion-compensating optical fiber of the fifth embodiment of thepresent invention to −40 ps/nm/km or less and −65 ps/nm/km or more at awavelength of 1.55 μm.

[0239] In addition, since the dispersion-compensating optical fiber ofthe fifth embodiment of the present invention compensates positivechromatic dispersion and positive dispersion slope of a single-modeoptical fiber for transmission, it is required to have negativechromatic dispersion slope. Although dependent on the characteristics ofthe single-mode optical fiber for transmission, it preferably has anegative chromatic dispersion slope within the range of, for example,−0.22 to −0.11 ps/nm²/km in the used wavelength band.

[0240] In addition, since the dispersion-compensating optical fiber ofthe fifth embodiment of the present invention is able to enlarge Aeff to18 μm² or more, and preferably 20 μm² or more, at a wavelength of 1.55μm, this is preferable from the viewpoint of controlling non-linearoptical effects. Although there is no particular restriction on theupper limit of Aeff, it should be about 30 μm² from the viewpoint of thebalance with other characteristics.

[0241] Furthermore, Aeff is defined by the previously mentionedexpression.

[0242] In addition, bending loss is the value obtained under conditionsof a bending radius (2R) of 20 mm at a wavelength of 1.55 μm, and in thefifth embodiment of the present invention, a dispersion-compensatingoptical fiber is obtained having bending loss is 50 dB/m or less, andpreferably 20 dB/m or less. If bending loss exceeds 50 dB/m, there arecases in which transmission loss deteriorates due to micro-bendingapplied during manufacturing or laying, etc.

[0243] In addition, since the dispersion-compensating optical fiber ofthe fifth embodiment of the present invention is a single-mode opticalfiber, it is required to have a cutoff wavelength that substantiallyallows single-mode propagation. Although the value according to the2m-method recommended in CCITT-G.652 is typically used for the cutoffwavelength, in the actual state of a long fiber, single-mode propagationcan be performed even if the value determined by the 2m-method is longerthan 1.55 μm. Thus, it is necessary to design the optical fiber so thata suitable cutoff wavelength is obtained according to the used lengthand other actual conditions of use.

[0244]FIGS. 6A through 6C are graphs comparing the dispersioncompensation characteristics for each wavelength range between thesegment core dispersion-compensating optical fiber of the fifthembodiment of the present invention, and a dispersion-compensatingoptical fiber provided with a W-shaped refractive index distributionpattern shown in FIG. 2. Furthermore, in the W-shaped refractive indexdistribution pattern, characteristics as the compensating optical fiberare also obtained by adjustment of structural parameters.

[0245] The width of residual dispersion of the vertical axis of thegraphs refers to the value (d1) determined by composing a hybridtransmission line by combining with a single-mode optical fiber fortransmission provided with the characteristics shown in Table 5-1 below,measuring the relationship between wavelength and chromatic dispersionin the manner of the graph shown, for example, in FIG. 7, andsubtracting the minimum value from the maximum value of chromaticdispersion for each wavelength range.

[0246] The plotted points of the graphs are the results of fabricating aplurality of segment core types and W-shaped types by changing thestructural parameters, constructing hybrid transmission lines andmeasuring the values, respectively.

[0247] These dispersion-compensating optical fibers have a cutoffwavelength that substantially allows single-mode propagation over thesewavelength ranges, have bending loss of 50 dB/m or less, and are able tosufficiently withstand lateral pressure.

[0248] In addition, the used length of the dispersion-compensatingoptical fibers was designed so that chromatic dispersion becomes zero ata wavelength of 1.55 μm throughout the entire hybrid transmission line.In addition, the ratio of the length of each dispersion-compensatingoptical fiber to the length of the single-mode optical fiber fortransmission was designed to be equal. TABLE 5-1 Transmission chromaticDispersion Bending loss dispersion slope 2mγc Aeff loss (dB/km)(ps/nm/km) (ps/nm²/km) (μm) (μm²) (dB/m) 0.186 +20.2 +0.062 1.45 12014.4

[0249] Furthermore, 2mλc in the table refers to the value of cutoffwavelength as measured according to the 2m-method.

[0250] The smaller the width of residual dispersion, the smaller thewavelength dependency of chromatic dispersion of the hybrid transmissionline. Namely, this means that chromatic dispersion and chromaticdispersion slope of a single-mode optical fiber for transmission iscompensated over a broad wavelength band, resulting in superiordispersion compensation characteristics.

[0251] The horizontal axis indicates the Aeff of thedispersion-compensating optical fiber at a wavelength of 1.55 μm, andthe larger this value, the more preferable it is from the viewpoint ofsuppressing non-linear effects.

[0252] Thus, it is preferable that an optical fiber be provided withcharacteristics plotted in the lower right corner of the graphs.

[0253] When comparing these graphs, points representing thecharacteristics of the segment core dispersion-compensating opticalfiber are distributed farther down and to the right than the W-shapeddispersion-compensating optical fiber. Accordingly, it can be seen thatan optical fiber is obtained that has superior dispersion compensationcharacteristics over a broad wavelength band as well as a large Aeff.

[0254] In addition, it is preferable that the dispersion-compensatingoptical fiber of the fifth embodiment of the present invention satisfythe conditions of (A) through (D) below for its structural parameters:

[0255] (A) 0.95≦Δ₁≦1.35;

[0256] (B) −3.5=V2/V1<0 and 0.5≦3/V1≦4.5 when V1=Δ₁, V2=Δ₂×{(b/a)²−1}and V3=Δ₃×{(c/a)²−(b/a)^(2};)

[0257] (C) α, expressed with α=−y{x−1}/Δ₁, is such that 0.10≦α≦0.45 whenx=C-band y=Δ ₃/Δ₂; and,

[0258] (D) the value resulting from dividing chromatic dispersion slopeat a wavelength of 1.55 μm by chromatic dispersion is 0.0025 nm¹ or moreand 0.0035 nm⁻¹ or less.

[0259] With respect to (A) above, if Δ₁ exceeds 1.35, Aeff becomessmaller, and if less than 0.95, dispersion compensation effects diminishover a broad wavelength band.

[0260] With respect to (B) above, if V2/V1 is less than −3.5, theproblem results in which transmission loss increases. In addition, ifV3/V1 exceeds 4.5, the cutoff wavelength becomes longer, and whetherV2/V1 or V3/V1 is too large or too small, it is no longer possible tocompensate chromatic dispersion slope.

[0261] (C) above is a condition for compensating chromatic dispersionover a broad wavelength band, and if α is too large, compensation is nolonger possible over a broad wavelength band, while if α is too small,bending loss increases thereby weakening the resistance tomicro-bending.

[0262] (D) above is a range that is nearly equal to the value obtainedby dividing the chromatic dispersion slope by chromatic dispersion of atypical single-mode optical fiber for transmission targeted forcompensation, and is a condition for compensation of chromaticdispersion of this single-mode optical fiber for transmission over abroad wavelength band.

[0263] Furthermore, even if the above conditions of (A) through (D) aresatisfied, a dispersion-compensating optical fiber of the fifthembodiment of the present invention provided with preferredcharacteristics as indicated above cannot always be obtained. This isbecause the dispersion-compensating optical fiber of the fifthembodiment of the present invention is preferably obtained by combiningand selecting a plurality of suitable structural parameters among thosethat satisfy (A) through (D) by trial and error. Consequently, it wasdecided to specify the dispersion-compensating optical fiber of thefifth embodiment of the present invention according to refractive indexdistribution pattern and characteristic values. It goes without sayingthat characteristics allowing compensation of positive chromaticdispersion of a single-mode optical fiber for transmission, whichincludes a 1.3 μm single-mode optical fiber, typically used over thistype of broad wavelength band cannot be obtained with a conventionaldispersion-compensating optical fiber.

[0264] More specifically, the dispersion-compensating optical fiber ofthe fifth embodiment of the present invention is able to compensatechromatic dispersion of, for example, a single-mode optical fiber fortransmission like that indicated below.

[0265] Namely, this dispersion-compensating optical fiber has an Aeff of40 μm² or more, positive chromatic dispersion and a cutoff wavelengththat substantially allows single-mode propagation at a wavelength of1.55 μm.

[0266] By then constructing a hybrid transmission line by combining witha single-mode optical fiber for transmission, its overall chromaticdispersion can be made to be −0.5 ps/nm/km or more and +0.5 ps/nm/km orless over a used wavelength band of a continuous range of 0.06 μm ormore selected from the wavelength range of 1.45-1.63 μm.

[0267] The used length of the dispersion-compensating optical fiber in ahybrid transmission line varies according to the chromatic dispersionand used length of the single-mode optical fiber for transmission.

[0268] For example, in the compensation of a typical single-mode opticalfiber for transmission having chromatic dispersion per unit length at1.55 μm of +16 ps/nm/km to +18 ps/nm/km, by using thedispersion-compensating optical fiber of the fifth embodiment of thepresent invention at a ratio of about ⅓ to ⅕ the length of thissingle-mode optical fiber for transmission, a hybrid transmission linecan be constructed that is provided with low chromatic dispersion over abroad wavelength band as described above.

[0269] More preferably, when the dispersion-compensating optical fiberof the fifth embodiment of the present invention is combined with asingle-mode optical fiber for transmission having an Aeff of 70 μm² ormore and chromatic dispersion of +16 ps/nm/km or more and +22 ps/nm/kmor less at a wavelength of 1.55 μm, the chromatic dispersion of theentire hybrid transmission line is −0.5 ps/nm/km or more and +0.5ps/nm/km or less, and preferably −0.2 ps/nm/km or more and +0.2 ps/nm/kmor less, at a used wavelength band of a continuous range of 0.06 μm ormore, and preferably 0.10 μm or more, selected from the wavelength rangeof 1.45-1.63 μm.

Embodiments

[0270] To begin with, the following provides an explanation ofembodiments as claimed in a first embodiment of the present invention.

[0271] (Embodiments as Claimed in a First Embodiment)

[0272] <Embodiment 1-1>

[0273] Five types (Nos. 1 through 5) of dispersion-compensating opticalfibers having the segment core refractive index distribution patternshown in FIG. 1 were fabricated followed by evaluation of theircharacteristics.

[0274] The optical characteristics of the dispersion-compensatingoptical fibers of Nos. 1 through 5 consisting of b/a, w/a, Δ₁, Δ₂ and Δ₃are shown in Table 1-1.

[0275] Furthermore, the cutoff wavelength (λ_(c)) is the value measuredaccording to the 2m-method of CITT. In addition, MFD is the mode fielddiameter. TABLE 1-1 Dispersion Measured Loss Dispersion slope FOM Wave(ps/ (ps/ (ps/ (ps/ Length nm/ nm nm²/ MFD λc nm/ Aeff No b/a w/a Δ₁ Δ₂Δ₃ (μm) km) km) km) (μm) (μm) dB) (μm²) 1 3 0.5 1 −0.4 1.0 1.55 0.28−54.6 −0.16 5.64 1.56 195 26.6 1.63 0.27 −61.7 −0.15 6.22 1.56 247 30.22 2.9 0.7 1 −0.4 0.7 1.55 0.3 −55.1 −0.16 5.65 1.54 184 26.5 1.63 0.28−62.1 −0.15 6.27 1.54 222 30.5 3 3 1 1.2 −0.4 0.5 1.55 0.32 −69.2 −0.205.22 1.41 218 21.9 1.63 0.29 −79.2 −0.19 5.84 1.41 273 28.1 4 3 1 1.2−0.4 0.6 1.55 0.32 −69.8 −0.21 5.23 1.67 218 22.5 1.63 0.29 −77.0 −0.195.86 1.67 265 27.5 5 3 1 1.2 −0.4 0.4 1.55 0.31 −71.6 −0.20 5.24 1.15231 21.9 1.63 0.30 −82.5 −0.20 5.88 1.15 273 27.1

[0276] According to Table 1-1, all of the dispersion-compensatingoptical fibers satisfied the characteristics of the first embodiment ofthe present invention.

[0277] Continuing, when a 1.3 μm single-mode optical fiber (chromaticdispersion of +17 ps/nm/km and dispersion slope of 0.06 ps/nm²/km at1.55 μm) was compensated using these dispersion-compensating opticalfibers, the results shown in Table 1-2 were obtained, and chromaticdispersion and dispersion slope of the 1.3 μm single-mode optical fiberwere able to be compensated over a range of 1.53-1.63 μm. TABLE 1-2 Useddistance (km) Overall Overall Dispersion- chromatic dispersionMeasurement compensating dispersion slope (ps/ wavelength 1.3 μm SMFoptical fiber (ps/nm/km) nm²/km) No. 1 1.53 μm 10 3.36 −0.29 −0.003 1.63μm −0.50 −0.008 No. 2 1.53 μm 10 3.35 −0.38 −0.002 1.63 μm 0.45 0.008No. 3 1.53 μm 10 2.65 −0.34 −0.005 1.63 μm 0.33 0.008 No. 4 1.53 μm 102.68 −0.49 −0.004 1.63 μm 0.50 0.007 No. 5 1.53 μm 10 2.55 −0.30 −0.0021.63 μm 0.30 0.008

[0278] <Comparative Example 1-1>

[0279] A dispersion-compensating optical fiber of the prior art havingthe matched cladding refractive index distribution pattern, which isprovided with a core and a cladding provided around an outer peripheryof the core, was fabricated. Furthermore, a relative refractive indexdifference between the core and the cladding was set at 2.5%. Theoptical characteristics of the resulting dispersion-compensating opticalfiber are shown in Table 1-3. TABLE 1-3 Fiber structure Matched claddingpattern Measurement wavelength 1.55 μm 1.63 μm Loss 0.37 dB/km 0.35dB/km Dispersion value −75 ps/nm/km −69.5 ps/nm/km Dispersion slope+0.09 ps/nm²/km +0.10 ps/nm²/km MFD 4.4 μm 4.8 μm FOM 202 ps/nm/dB 198ps/nm/dB Aeff 14.8 μm² 17.7 μm²

[0280] <Comparative Example 1-2>

[0281] A dispersion-compensating optical fiber having the W-shapedrefractive index distribution pattern shown in FIG. 2 was fabricated.Furthermore, 2 a ₁, was set at 2.5, b₁/a₁ at 2.5, Δ₁₁ at 0.35 and Δ₁₂ at2.5. Optical characteristics are shown in Table 1-4. Furthermore,bending loss for light at 1.63 μm was large and light was unable to betransmitted. TABLE 1-4 Fiber structure W-shaped Measurement wavelength1.55 μm Loss 0.45 dB/km Dispersion value −138 ps/nm/km Dispersion slope−0.49 ps/nm²/km MFD 3.9 μm FOM 300 ps/nm/dB Aeff 12 μm²

[0282] According to Tables 1-3 and 1-4, the values of Aeff for both ofthe dispersion-compensating optical fibers of Comparative Examples 1-1and 1-2 were small.

[0283] The following provides an explanation of embodiments as claimedin a second embodiment of the present invention.

[0284] (Embodiments as Claimed in a Second Embodiment)

[0285] <Comparative Example 2-1>

[0286] A cylindrical porous body provided with a first layer comprisedof GeO₂-doped quartz glass and a second layer comprised of SiO₂ providedaround an outer periphery of said first layer was fabricated by a knownmethod such as VAD or MCVD. Furthermore, the ratio of the diameter ofthe second layer to the diameter of the first layer was 4.0.

[0287] This porous body was subjected to dehydration treatment with Heand chlorine gas in an atmosphere of about 1000° C. followed bysimultaneously doping with fluorine and transparent vitrification in anatmosphere of He at 5 l/min and SiF₄ at 1 l/min to fabricate a rod.

[0288] This rod was then drawn to form a core base material, and aporous body composed of SiO₂ for cladding was attached around itfollowed by dehydration treatment with He and chlorine gas in anatmosphere of about 1000° C and then transparent vitrification in a Heatmosphere to fabricate a fiber base material. Subsequently, the fiberbase material was drawn to produce a dispersion-compensating opticalfiber having the W-shape refractive index distribution pattern shown inFIG. 2.

[0289] The optical characteristics of this dispersion-compensatingoptical fiber at a used wavelength of 1.55 μm are shown in Table 2-1.MFD in Table 2-1 refers to the mode field diameter, while wavelengthrefers to the wavelength at which optical characteristics were measured(used wavelength). TABLE 2-1 No. 1 Δ₁₁ 0.8 Δ₁₂ −0.48 a1:b1 1:2Wavelength 1.55 Core radius (μm) 4.5 Cutoff wavelength (μm) 0.89 Aeff(μm²) 25.1 MFD (μm) 5.7 Bending loss (dB/m) 31.3 2R = 20 mm Chromaticdispersion (ps/nm/km) −16.2 Dispersion slope (ps/nm²/km) −0.057Dispersion slope compensation rate (%) 100

[0290] Dispersion slope compensation rate in Table 2-1 refers to theratio of the absolute value of dispersion slope per unit length of thedispersion-compensating optical fiber to the absolute value ofdispersion slope per unit length of a 1.3 μm single-mode optical fiber.

[0291] Furthermore, the typical chromatic dispersion at 1.55 μm of a 1.3μm single-mode optical fiber is +17 ps/nm/km, and the typical dispersionslope is +0.060 ps/nm²/km.

[0292] According to Table 2-1, since the chromatic dispersion of thisdispersion-compensating optical fiber is −16.2 ps/nm/km, for the sake ofconvenience, the chromatic dispersion of the 1.3 μm single-mode opticalfiber was assumed to be zero, and the used length of thedispersion-compensating optical fiber that allowed complete compensationwas 1.05 km.

[0293] Since the dispersion slope of this dispersion-compensatingoptical fiber is −0.057 ps/nm²/km, the dispersion slope of thedispersion-compensating optical fiber at this used length (1.05 km) is−0.060 ps/nm², and it was found that the dispersion slope of 1 km of theabove 1.3 μm single-mode optical fiber is able to be completelycompensated.

[0294] Next, this dispersion-compensating optical fiber was connected toa 1.3 μm single-mode optical fiber at the same used length ratio toconstruct a transmission path over a total length of 45 km.

[0295] the Aeff of the dispersion-compensating optical fiber used in thelatter half of this transmission path is 25.1 μm² and the Aeff is small,deterioration of transmission characteristics due to non-linear effectsis large, and it was found to be difficult to increase transmissioncapacity, increase transmission distance and apply this transmissionpath to long-distance, large capacity transmission.

[0296] <Embodiment 2-1>

[0297] Rods were fabricated in the same manner as the comparativeexamples while changing the size of each layer and so forth. Next, therods were drawn, a porous body composed of GeO₂-doped quartz glass andSiO₂ for the ring core portion and cladding were was attached around therods, and the rods were subjected to dehydration treatment with He andchlorine gas in an atmosphere of about 1000° C. followed by transparentvitrification in an He atmosphere to obtain base materials. The basematerials were then drawn to produce dispersion-compensating opticalfibers provided with the refractive index distribution shown in FIG. 1.

[0298] The structural parameters and optical characteristics at a usedwavelength of 1.55 μm of the resulting dispersion-compensating opticalfibers are shown in Tables 2-2 and 2-3. TABLE 2-2 No. 2 3 4 5 6 Δ₁ 0.650.75 0.70 0.70 0.80 Δ₂ −0.50 −0.35 −0.40 −0.50 −0.30 Δ₃ 0.50 0.70 0.450.65 0.80 a:b:c 1:2.1:2.8 1:2.7:3.3 1:2.3:3.2 1:2.2:2.8 1:2.5:3.0Wavelength 1.55 1.55 1.55 1.55 1.55 Core radius 7.8 7.9 8.3 7.6 7.1 (μm)Cutoff 1.57 1.69 1.69 1.64 1.54 wavelength (μm) Aeff (μm²) 38.3 35.536.4 35.3 30.9 MFD (μm) 6.7 6.5 6.6 6.4 6.1 Bending 38.5 39.4 22.4 12.98.2 loss (dB/m) 2R = 20 mm Chromatic −18.4 −30.8 −22.1 −18.8 −24.8dispersion (ps/nm/km) Dispersion −0.05 −0.09 −0.06 −0.06 −0.06 slope(ps/nm²/km)

[0299] TABLE 2-3 No. 2 3 4 5 6 Dispersion 77 83 77 90 69 slopecompensation rate (%) Used length (km) 0.92 0.55 0.77 0.90 0.69Chromatic 0 0 0 0 0 dispersion of entire transmission path (ps/nm/km)Dispersion +0.014 +0.010 +0.014 +0.006 +0.019 slope of entiretransmission path (ps/nm²/km)

[0300] According to the results shown in Tables 2-2 and 2-3, all of thedispersion-compensating optical fibers of numbers 2 through 6 as claimedin a second embodiment of the present invention were able to compensatechromatic dispersion and dispersion slope of a standard 1.3 μmsingle-mode optical fiber.

[0301] Furthermore, for the sake of convenience, the used length inTable 2-3 refers to the used length of the dispersion-compensatingoptical fiber that allows complete compensation assuming the chromaticdispersion of the 1.3 μm single-mode optical fiber of 1 km to be zero.

[0302] In addition, chromatic dispersion or dispersion slope of theentire transmission path refers to the chromatic dispersion ordispersion slope per unit length when a dispersion-compensating opticalfiber of the above used length was connected to a 1 km 1.3 μmsingle-mode optical fiber.

[0303] In addition, a transmission path was constructed having a totallength of 45 km by connecting a 1.3 μm single-mode optical fiber anddispersion-compensating optical fiber at the same used length ratio.Since the Aeff of the dispersion-compensating optical fiber used in thelatter half of this transmission path was large at 30 μm² or more, therewas little deterioration of transmission characteristics due tonon-linear effects, and this transmission path was found to be able toperform long-distance, large capacity transmission by increasing thetransmission capacity or increasing the transmission distance.

[0304] In addition, the dispersion-compensating optical fiber allowssingle-mode propagation at the used length used for the transmissionpath.

[0305] The following provides an explanation of embodiments as claimedin a third embodiment of the present invention.

[0306] (Embodiments as Claimed in a Third Embodiment)

[0307] <Embodiment 3-1 and Comparative Example 3-1>

[0308] A fiber base material was fabricated in accordance with knownmethods such as VAD, MCVD and PCVD after which it was drawn to producefive types of dispersion-compensating optical fibers (A through E:Embodiments, F: Comparative Example).

[0309] These dispersion-compensating optical fibers were provided withthe refractive index distribution pattern shown in FIG. 1, and thevalues for Δ₁, Δ₂, Δ₃, b/a and c/a were set to the values shown in Table3-1 for each of the dispersion-compensating optical fibers.

[0310] In addition, the central core portion 1 and the ring core portion3 were formed from germanium-doped quartz glass, the intermediate coreportion 2 was formed from fluorine-doped quartz glass, and the cladding5 was formed from fluorine-doped quartz glass. The fluorineconcentration of the cladding 5 was such that the relative refractiveindex difference based on the refractive index of pure quartz glass was0.1%.

[0311] The conditions for drawing consisted of a drawing speed of 300m/min, drawing tension of 250 g and heating temperature of 2000° C.

[0312] However, although drawing was able to be performed withoutincident for the dispersion-compensating optical fibers of embodiments Athrough E, due to the high loss of the dispersion-compensating opticalfiber of comparative example F having a large value for Δ₁, drawingconditions were changed to a drawing speed of 300 m/min, drawing tensionof 350 g and heating temperature of 1900° C. However, there was aproblem with mechanical strength.

[0313] The optical characteristics of these dispersion-compensatingoptical fibers are collectively shown in Table 3-1. TABLE 3-1 Core UsedTransmission radius Wavelength Aeff loss No. Δ1 (%) Δ2 (%) Δ3 (%) b/ac/b (μm) (μm) (μm²) (dB/km) A 1.0 −0.40 0.90 3.0 3.5 6.7 1.55 25.4 0.28B 1.0 −0.40 1.0 3.0 3.5 6.8 1.55 26.6 0.29 C 1.1 −0.40 0.80 3.0 3.5 6.21.55 22.9 0.28 D 1.0 −0.40 0.70 2.9 3.6 7.0 1.55 26.5 0.28 E 1.2 −0.400.40 3.0 4.0 6.7 1.55 20.8 0.30 F 2.3 −0.40 0 4.0 — 4.1 1.55 12.2 0.40Chromatic Bending loss dispersion Dispersion slope (dB/m) Cutoffwavelength (ps/nm/km) (ps/nm²/km) 2R = 20 mm (μm) −52.8 −0.17 26.1 1.55−54.6 −0.16 16.5 1.53 −56.8 −0.16 21.8 1.60 −55.1 −0.16 15.3 1.61 −63.4−0.19 11.3 1.59 −127.7 −0.32 8.3 0.76

[0314] The transmission loss of dispersion-compensating optical fibers Athrough E was low ranging from 0.25 to 0.30 dB/km, and Aeff values werelarge at 20 μm² or more, making these optical fibers suitable forlarge-capacity and long-distance transmission.

[0315] On the other hand, even if the dispersion-compensating opticalfiber of F was drawn at a level at which problems with mechanicalstrength occur, transmission loss was large at 0.40 dB/m. In addition,Aeff was small.

[0316] Furthermore, cutoff wavelength indicates the value determined bythe 2m-method, and in the long states in optical fibers are normallyused, values were obtained that were able to guarantee single-modepropagation.

[0317] <Embodiment 3-2>

[0318] A hybrid transmission line was constructed by arranging 30 km ofthe following single-mode optical fiber in the former stage, andconnecting 11.6 km of the dispersion-compensating optical fiber of Aproduced in Embodiment 3-1 in the latter stage.

[0319] Namely, the single-mode optical fiber was produced by drawing afiber base material fabricated in accordance with known methods such asVAD and MCVD.

[0320] This single-mode optical fiber had the refractive indexdistribution pattern shown in FIG. 2, the central core was composed ofgermanium-doped quartz glass, the intermediate core portion was composedof fluorine-doped quartz glass, and the cladding was composed of purequartz glass.

[0321] In addition, Δ₁₁ and Δ₁₂ were 0.24% and −0.05%, respectively, aand b were 6.6 μm and 26.5 μm, respectively, and the cladding outerdiameter was 125 μm.

[0322] The characteristics of this single-mode optical fiber are shownin Table 3-2. TABLE 3-2 Bending Used Transmission Chromatic Dispersionloss Cutoff Wavelength Aeff loss dispersion slope (dB/m) wavelength Δ₁₁Δ₁₂ b1/a1 (μm) (μm²) (dB/km) (ps/nm/km) (ps/nm²/km) 2R = 20 mn) (μm)0.24 −0.05 4.0 1.55 132 0.188 +20.4 +0.062 10.2 1.50

[0323] Chromatic dispersion at a wavelength of 1.55 μm was nearly zerofor this hybrid transmission line. In addition, dispersion slope was 0.1ps/nm²/km or less over a range of 1.53-1.63 μm, and the compensationrate of dispersion slope was nearly 100%.

[0324] Due to the large value of Aeff of the single-mode optical fiberof the former stage, this hybrid transmission line has lownon-linearity, and due to the action of the dispersion-compensatingoptical fiber, both chromatic dispersion and dispersion slope are small,resulting in the obtaining of satisfactory transmission characteristics.

[0325] In this third embodiment of the present invention, since therelative refractive index difference of the layer in which is providedwith the highest refractive index of the core is small, an optical fibercan be obtained having low loss by drawing at lower tension than that ofthe prior art. In addition, the occurrence of non-linear effects can besuppressed by enlarging Aeff.

[0326] In addition, the dispersion-compensating optical fiber of thethird embodiment of the present invention can be used to construct ahybrid transmission line suitable for wavelength division multiplexingtransmission or long-distance transmission by combining with asingle-mode optical fiber.

[0327] The following provides an explanation of embodiments as claimedin a fourth embodiment of the present invention.

[0328] (Embodiments as Claimed in a Fourth Embodiment)

[0329] <Embodiment 4-1>

[0330] Dispersion-compensating optical fibers provided with therefractive index distribution pattern shown in FIG. 3 were produced.

[0331] To begin with, a cylindrical porous body was fabricated by VADhaving a structure consisting of a GeO₂-doped core and SiO₂ cladding(cladding diameter/core diameter=2.0-2.2). However, the core andcladding referred to here do not correspond to the core and cladding inan actual dispersion-compensating optical fiber, but rather are namesused for the sake of convenience to indicate each layer of a two-layerstructure. In other words, the central portion is referred to as thecore (portion serving as the central core portion 21), and the portionsurrounding it is referred to as the cladding (portion serving as theintermediate core portion 2 2).

[0332] After subjecting this porous body to dehydration treatment withHe and chlorine gas in an atmosphere at 1000° C., fluorine doping andtransparent vitrification were simultaneously performed in an atmosphereof He at 5 l/min and SiF₄ at 1 l/min. The resulting rod was then drawnto form a core material, after which porous bodies comprised ofSiO₂-GeO₂ and SiO₂ were attached to the outside for the ring core andside ring core portions and for the cladding, respectively. This wasfollowed by dehydration treatment with He and chlorine gas in anatmosphere at 1000° C. and transparent vitrification in an He atmosphereto obtain fiber base materials having an outer diameter of 50 mm.

[0333] These fiber base materials were then drawn to producedispersion-compensating optical fibers having an outer diameter of 125μm. The drawing speed at this time was set at 300 m/min, drawing tensionto 200 g, and heating temperature to 1950° C.

[0334] In this Embodiment 4-1, five types of dispersion-compensatingoptical fibers having different structural parameters were producedusing a similar method. The structural parameters and opticalcharacteristics of each optical fiber are shown in Table 4-1 and Table4-2. TABLE 4-1 Aeff Core (μm²) Sample Δ₂₁ Δ₂₂ Δ₂₃ Δ₂₄ diameter @ 1550No. (%) (%) (%) (%) B/a c/a d/a (μm) nm 1 0.95 −0.52 0.10 −0.5 2.0 7.08.5 34.4 26.8 2 0.95 −0.52 0.12 −0.5 2.0 6.4 7.9 32.6 26.3 3 0.95 −0.440.12 −0.5 2.2 6.4 7.9 31.7 26.3 4 0.95 −0.48 0.14 −0.5 2.2 6.0 7.5 30.326.5 5 0.95 −0.50 0.22 −0.3 2.2 5.0 6.5 27.3 27.4

[0335] TABLE 4-2 Transmission Chromatic Dispersion Cutoff BendingBending loss dispersion slope wavelength in loss loss Sample (dB/km)(ps/nm/km) @ 1550 (ps/nm²/km) @ 1550 2 m-method (dB/km) (dB/km) No. @1550 nm nm nm (μm) @ 1550 nm @ 1630 nm 1 0.34 −54.0 −0.17 1.58 7.5 48 20.34 −49.2 −0.15 1.61 3.6 27 3 0.33 −49.0 −0.15 1.53 5.7 39 4 0.33 −54.0−0.17 1.55 6.5 44 5 0.34 −54.0 −0.16 1.65 3.8 34

[0336] According to the results of Table 4-1 and Table 4-2, non-lineareffects were able to be suppressed by enlarging Aeff. In addition, smallvalues were also obtained for bending loss at a wavelength of 1.63 μm.

[0337] Furthermore, there were no problems with mechanical strength forany of these dispersion-compensating optical fibers under theirrespective drawing conditions.

[0338] <Comparative Example 4-1>

[0339] Dispersion-compensating optical fibers provided with the segmentcore refractive index distribution pattern shown in FIG. 1 wereproduced.

[0340] To begin with, a cylindrical porous body was fabricated by VADhaving a structure consisting of a GeO₂-doped core and SiO₂ cladding(cladding diameter/core diameter=2.2-2.4). However, the core andcladding referred to here do not correspond to the core and cladding inan actual dispersion-compensating optical fiber, but rather are namesused for the sake of convenience to indicate each layer of a two-layerstructure. In other words, the central portion is referred to as thecore (portion serving as the central core portion 1), and the portionsurrounding it is referred to as the cladding (portion serving asintermediate core portion 2).

[0341] After subjecting this porous body to dehydration treatment withHe and chlorine gas in an atmosphere at 1000° C., fluorine doping andtransparent vitrification were simultaneously performed in an atmosphereof He at 5 l/min and SiF₄ at 1 l/min. The resulting rod was then drawnto form a core material, after which porous bodies comprised ofSiO₂—GeO₂ and SiO₂ were attached to the outside for the ring core andfor the cladding, respectively. This was followed by dehydrationtreatment with He and chlorine gas in an atmosphere at 1000° C. andtransparent vitrification in an He atmosphere to obtain an intermediatecore portion. Moreover, a porous body comprised of SiO₂ was attachedaround this followed by dehydration treatment with He and chlorine gasin an atmosphere at 1000° C. and then transparent vitrification in an Heatmosphere to produce fiber base materials having an outer diameter of50 mm.

[0342] These fiber base materials were then drawn to producedispersion-compensating optical fibers having an outer diameter of 125μm. The drawing speed at this time was set at 300 m/min, drawing tensionto 200 g, and heating temperature to 1950° C.

[0343] The optical characteristics at 1.55 μm of thesedepression-compensating optical fibers are shown in Tables 4-3 and 4-4.TABLE 4-3 Core Aeff Sample diameter (μm²) No. Δ₁ (%) Δ₂ (%) Δ₃ (%) b/ac/a (μm) @ 1550 nm 6 0.95 −0.50 0.24 2.2 4.2 23.9 27.5 7 0.95 −0.48 0.342.4 3.8 32.5 27.5 8 0.95 −0.58 0.62 2.4 3.2 20.0 27.6 9 0.95 −0.54 0.282.2 3.8 22.3 27.3 10  0.95 −0.58 0.30 2.2 3.6 21.4 26.5

[0344] TABLE 4-4 Transmission Chromatic Dispersion Cutoff BendingBending loss dispersion slope wavelength in loss loss Sample (dB/km)(ps/nm/km) @ 1550 (ps/nm²/km) @ 1550 2 m-method (dB/km) (dB/km) No. @1550 nm nm nm (μm) @ 1550 nm @ 1630 nm 6 0.33 −54 −0.17 1.64  8.8 81 70.32 −54 −0.17 1.60 11.0 100  8 0.35 −54 −0.15 1.57  9.7 88 9 0.34 −54−0.15 1.53 10.0 91 10  0.35 −54 −0.16 1.43 15.0 122 

[0345] The goal of these dispersion-compensating optical fibers was tohave an Aeff value or cutoff wavelength comparable to thedispersion-compensating optical fibers shown in Embodiment 4-1 and assmall a value as possible for bending loss.

[0346] According to the results of Table 4-3 and Table 4-4, bending lossin the vicinity of a wavelength of 1.55 μm was sufficiently small, andwas determined not to present a problem. However, bending loss at awavelength of 1.63 μm was large, and transmission loss was also found tobe large.

[0347] The following provides an explanation of embodiments as claimedin a fifth embodiment of the present invention.

[0348] (Embodiments as Claimed in a Fifth Embodiment)

[0349] (Embodiment 5-1)

[0350] A dispersion-compensating optical fiber having the refractiveindex distribution pattern shown in FIG. 1 was produced. Its opticalcharacteristics are shown in Table 5-2, and those characteristics werefound to be satisfactory. TABLE 5-2 Core diameter (c) (μm) Δ₁ Δ₂ Δ₃ b/ac/a 14.7  1.1 −0.56 0.12 2.2 4 Transmission Chromatic Dispersion lossdispersion slope 2m λc Aeff Bending (dB/km) (ps/nm/km) (ps/nm²/km) (μm)(μm²) loss (dB/m)  0.302 −54 −0.163 0.86 21.2 16.6

[0351] A hybrid transmission line was then constructed by combining 13.6km of this dispersion-compensating optical fiber with 36.4 km of thesingle-mode optical fiber for transmission provided with thecharacteristics shown in Table 5-1. Furthermore, the used lengths ofthese optical fibers were set so that the chromatic dispersion of theentire hybrid transmission line at a wavelength of 1.55 μm was zero.

[0352]FIG. 8A is a graph showing the relationship between wavelength andchromatic dispersion of the dispersion-compensating optical fiber andthe single-mode optical fiber for transmission, while FIG. 8B is a graphshowing the relationship between wavelength and chromatic dispersion ofthe hybrid transmission line.

[0353] A hybrid transmission line was able to be constructed having alow level of chromatic dispersion within a range of −0.15 to +0.1ps/nm/km over a broad range of about 0.1 μm extending from the C-band tothe L-band.

[0354] (Embodiment 5-2)

[0355] A dispersion-compensating optical fiber having the refractiveindex distribution pattern shown in FIG. 1 was produced. Its opticalcharacteristics are shown in Table 5-3, and those characteristics werefound to be satisfactory. TABLE 5-3 Core diameter (c) (μm) Δ₁ Δ₂ Δ₃ b/ac/a 13.2 1.1 −0.52 0.68 2.6 3.4 Transmission Chromatic Dispersion lossdispersion slope 2 m λc Aeff Bending (dB/km) (ps/nm/km) (ps/nm²/km) (μm)(μm²) loss (dB/m) 0.298 −54 −0.171 1.59 22.7 0.8

[0356] A hybrid transmission line was then constructed by combining 13.6km of this dispersion-compensating optical fiber with 36.4 km of thesingle-mode optical fiber for transmission provided with thecharacteristics shown in Table 5-1. Furthermore, the used lengths ofthese optical fibers were set so that the chromatic dispersion of theentire hybrid transmission line at a wavelength of 1.55 μm was zero.

[0357]FIG. 9A is a graph showing the relationship between wavelength andchromatic dispersion of the dispersion-compensating optical fiber andthe single-mode optical fiber for transmission, while FIG. 9B is a graphshowing the relationship between wavelength and chromatic dispersion ofthe hybrid transmission line.

[0358] A hybrid transmission line was able to be constructed having alow level of chromatic dispersion within a range of −0.4 to +0.4ps/nm/km over a broad range of about 0.115 μm extending from the S-bandto the C-band.

[0359] (Comparative Example 5-1)

[0360] A dispersion-compensating optical fiber having the W-shapedrefractive index distribution pattern shown in FIG. 2 was produced. Itsoptical characteristics are shown in Table 5-4, and thosecharacteristics were found to be satisfactory. TABLE 5-4 Core diameter(c) (μm) Δ₁₁ Δ₁₂ b₁/a₁ 7.9 1.24 −0.5 2.4 Transmission ChromaticDispersion loss dispersion slope 2 m λc Aeff Bending (dB/km) (ps/nm/km)(ps/nm²/km) (μm) (μm²) loss (dB/m) 0.311 −54 −0.161 0.83 18.1 20.9

[0361] A hybrid transmission line was then constructed by combining 13.6km of this dispersion-compensating optical fiber with 36.4 km of thesingle-mode optical fiber for transmission provided with thecharacteristics shown in Table 5-1. Furthermore, the used lengths ofthese optical fibers were set so that the chromatic dispersion of theentire hybrid transmission line at a wavelength of 1.55 μm was zero.

[0362]FIG. 10A is a graph showing the relationship between wavelengthand chromatic dispersion of the dispersion-compensating optical fiberand the single-mode optical fiber for transmission, while FIG. 10B is agraph showing the relationship between wavelength and chromaticdispersion of the hybrid transmission line.

[0363] Although chromatic dispersion of the entire transmission path wasable to be reduced to −0.3-0 ps/nm/km in the C-band, chromaticdispersion was on the order of several ps/nm/km in other wavelengthbands.

[0364] According to the results of the above embodiments and comparativeexamples, the embodiments as claimed in the fifth embodiment of thepresent invention were clearly able to compensate chromatic dispersionof a single-mode optical fiber for transmission over a broad wavelengthband, as well as suppress non-linear effects by enlarging Aeff.

What is claimed is:
 1. A dispersion-compensating optical fiber thatcompensates chromatic dispersion of a 1.3 μm single-mode optical fiberover the entire wavelength range of 1.53-1.63 μm wherein, chromaticdispersion at a wavelength of 1.55 μm is −50 ps/nm/km or less, thedispersion slope is negative over the entire wavelength range of1.53-1.63 μm, a cutoff wavelength is provided at which there issubstantially single-mode propagation over the entire wavelength rangeof 1.53-1.63 μm, bending loss is 30 dB/m or less over the entirewavelength range of 1.53-1.63 μm, effective area is 20 μm² or more overthe entire wavelength range of 1.53-1.63 μm, and the absolute value ofchromatic dispersion during compensation of the chromatic dispersion ofa 1.3 μm single-mode optical fiber serving as the target of compensationis 0.5 ps/nm/km or less over the entire wavelength range of 1.53-1.63μm.
 2. A dispersion-compensating optical fiber according to claim 1provided with a core and a cladding provided around an outer peripheryof said core, said core comprising a central core portion having arefractive index higher than said cladding, an intermediate core portionprovided around an outer periphery of said central core portion andhaving a refractive index lower than said cladding, and a ring coreportion provided around an outer periphery of said intermediate coreportion and having a refractive index higher than said cladding.
 3. Adispersion-compensating optical fiber according to claim 2 wherein, whenan outer diameter of the central core portion is taken to be 2 a, aninner diameter of the ring core portion to be 2 b, the width of the ringcore portion to be w, the relative refractive index difference betweenthe cladding and the intermediate core portion to be Δ₂, the relativerefractive index difference between the cladding and the ring coreportion to be Δ₃, and the relative refractive index difference betweenthe cladding and the central core portion to be Δ₁, 2.5≦b/a≦5.0,0.3≦w/a≦1.7, Δ₂ is −0.2 to −0.5%, Δ₃ is 0.1 to 1.3% and Δ₁ is 1.5% orless.
 4. A dispersion-compensating optical fiber wherein, in a usedwavelength band selected from 1.53 to 1.63 μm effective area is 30 μm²or more, bending loss is 40 dB/m or less, chromatic dispersion is −40 to−10 ps/nm/km, an absolute value of chromatic dispersion over an entiretransmission path connected to a single-mode optical fiber fortransmission provided with positive chromatic dispersion is 4.0 ps/nm/kmor less, the absolute value of dispersion slope over the entiretransmission path is 0.03 ps/nm² /km or less, and a cutoff wavelength isprovided that allows substantially single-mode propagation at the usedlength used in the above transmission path.
 5. A dispersion-compensatingoptical fiber according to claim 4 provided with a core and a claddingprovided around an outer periphery of said core, said core is providedwith a central core portion having a refractive index higher than saidcladding, an intermediate core portion provided around an outerperiphery of said central core portion and having a refractive indexlower than said cladding, and a ring core portion provided around anouter periphery of said intermediate core portion and having arefractive index higher than said cladding, and said cladding providedwith a refractive index that is equal to or less than the refractiveindex of pure quartz.
 6. A dispersion-compensating optical fiberaccording to claim 5 wherein, when the outer diameter of the centralcore portion is taken to be 2 a, the outer diameter of the intermediatecore portion to be 2 b, and the outer diameter of the ring core portionto be 2 c, 2.0≦b/a≦3.0 and 2.5≦c/a≦4.0; and, the relative refractiveindex difference of the central core portion relative to the cladding Δ₁is 0.6 to 0.9%, the relative refractive index difference of theintermediate core portion relative to the cladding Δ₂ is −0.30 to−0.50%, and the relative refractive index difference of the ring coreportion relative to the cladding Δ₃ is 0.4 to 0.9%.
 7. Adispersion-compensating optical fiber wherein, a core and a claddingprovided around an outer periphery of said cladding are provided, saidcore is comprising of a central core portion having a refractive indexhigher than said cladding, an intermediate core portion provided aroundan outer periphery of said central core portion having a refractiveindex lower than said cladding, and a ring core portion provided aroundan outer periphery of said intermediate core portion having a refractiveindex higher than said cladding, when radii and relative refractiveindex differences based on the cladding of the central core portion, theintermediate core portion and the ring core portion are expressed as(a,Δ₁), (b,Δ₂) and (C,Δ₃), respectively, a is 2-3 μm, Δ₁ is 0.9 to 1.5%,Δ₂ is −0.30 to −0.45%, Δ₃ is 0.2 to 1.2%, b/a is 2.0 to 3.5, and c/a is3.0 to 5.0, in a used wavelength band selected from 1.53 to 1.63 μm,effective area is 20 μm² or more, bending loss is 40 dB/m or less,chromatic dispersion is −65 to −45 ps/nm/km, and a cutoff wavelength isprovided that allows substantially single-mode propagation, and thecompensation rate of dispersion slope when compensating a single-modeoptical fiber, at a length at which chromatic dispersion of saidsingle-mode optical fiber having a zero dispersion wavelength at awavelength shorter than the above used wavelength band can becompensated to zero, is 80-120%.
 8. A dispersion-compensating opticalfiber according to claim 7 wherein, said cladding is composed of quartzglass containing dopant.
 9. A hybrid transmission line combining asingle-mode optical fiber and a dispersion-compensating optical fiberaccording to claim 7 that compensate the chromatic dispersion anddispersion slope of said single-mode optical fiber.
 10. A hybridtransmission line according to claim 9 wherein, said single-mode opticalfiber is provided with a core and a cladding provided around an outerperiphery of said core, said core has a central core portion, anintermediate core portion provided around an outer periphery of saidcentral core portion and having a refractive index lower than said coreportion, and a cladding provided around an outer periphery of saidintermediate core portion having a refractive index higher than saidintermediate core portion and lower than said central core portion, inthe used wavelength band of said dispersion-compensating optical fiber,the effective area is 120 μm² or more, and a cutoff wavelength isprovided that substantially allows single-mode propagation.
 11. Adispersion-compensating optical fiber wherein, a core and a claddingprovided around an outer periphery of said core are provided, said coreis provided with a central core portion having a refractive index higherthan said cladding, an intermediate core portion provided around anouter periphery of said central core portion having a refractive indexlower than said cladding, a ring core portion provided around an outerperiphery of said intermediate core portion having a refractive indexhigher than said cladding, and a side ring core portion provided aroundan outer periphery of said ring core portion having a refractive indexlower than said cladding, in a used wavelength band selected from 1.45to 1.63 μm, chromatic dispersion is −70 to −45 ps/nm/km, chromaticdispersion slope is negative, effective area is 20 μm²or more, and acutoff wavelength is provided that allows substantially single-modepropagation, and when a single-mode optical fiber is compensated at alength at which chromatic dispersion of this single-mode optical fiberhaving zero dispersion at a wavelength shorter than said used wavelengthband can be compensated to zero, the compensation rate of dispersionslope defined as RDS(DCF)/RDS (single-mode optical fiber)×100, when thevalue obtained by dividing dispersion slope of the single-mode opticalfiber by chromatic dispersion of the single-mode optical fiber is takento be RDS (single-mode optical fiber), and the value obtained bydividing dispersion slope of the dispersion-compensating optical fiberby chromatic dispersion of the dispersion-compensating optical fiber istaken to be RDS (DCF), is 80-120%, and bending loss at a wavelength of1.63 μm is 50 dB/m or less.
 12. A dispersion-compensating optical fiberaccording to claim 11 wherein, the relative refractive index differencebetween said side ring core portion and said cladding is −0.50 to 0.00%,and the value of the normalized frequency, defined as (relativerefractive index difference between said side ring core portion and saidcladding)×{(radius of said side ring core portion/radius of said centralcore portion)²−(radius of said ring core portion/radius of said centralcore portion)²}, is −15.0 to 0.0.
 13. A dispersion-compensating opticalfiber according to claim 11 wherein, the relative refractive indexdifference between said central core portion and said cladding is 0.90to 1.30%.
 14. A dispersion-compensating optical fiber according to claim11 wherein, the relative refractive index difference of the central coreportion between the cladding is 0.90 to 1.00%, and the effective area is25 μm² or more.
 15. A hybrid transmission line combining adispersion-compensating optical fiber according to claim 11 with asingle-mode optical fiber having an effective area of 70 μm² or more ata used wavelength band selected from 1.45-1.63 μm, and having a cutoffwavelength that substantially allows single-mode propagation at the usedband.
 16. A dispersion-compensating optical fiber wherein, a core and acladding provided around an outer periphery of said core are provided,said core is provided with a central core portion having a refractiveindex higher than said cladding, an intermediate core portion providedaround an outer periphery of said central core portion having arefractive index lower than said cladding, and a ring core portionprovided around an outer periphery of said intermediate core portionhaving a refractive index higher than said cladding, at a wavelength of1.55 μm, chromatic dispersion is −40 ps/nm/km or less and −65 ps/nm/kmor more, chromatic dispersion slope is negative, effective area is 18μm² or more, bending loss is 50 dB/m or less, and a cutoff wavelength isprovided that allows substantially single-mode propagation, and thechromatic dispersion of a hybrid transmission line, combined with asingle-mode optical fiber for transmission in which, at a wavelength of1.55 μm, effective area is 40 μm² or more, chromatic dispersion ispositive, and a cutoff wavelength is provided that allows substantiallysingle-mode propagation, is −0.5 ps/nm/km or more and +0.5 ps/nm/km orless at a used wavelength band over a continuous range of 0.06 μm ormore selected from a wavelength range of 1.45-1.63 μm.
 17. Adispersion-compensating optical fiber according to claim 16 wherein, theeffective area of said single-mode optical fiber for transmission at awavelength of 1.55 μm is 70 μm²or more, and chromatic dispersion is +16ps/nm/km or more and +22 ps/nm/km or less at a wavelength of 1.55 μm.18. A dispersion-compensating optical fiber according to claim 17wherein, the chromatic dispersion of a hybrid transmission line is −0.5ps/nm/km or more and +0.5 ps/nm/km or less at a used wavelength bandover a continuous range of 0.10 μm or more selected from a wavelengthrange of 1.45-1.63 μm.
 19. A dispersion-compensating optical fiberaccording to claim 18 wherein, the chromatic dispersion of a hybridtransmission line is −0.2 ps/nm/km or more and +0.2 ps/nm/km or less ata used wavelength band over a continuous range of 0.10 μm or moreselected from a wavelength range of 1.45-1.63 μm.
 20. Adispersion-compensating optical fiber according to claim 16 wherein,when the radii and relative refractive index differences based on thecladding of the central core portion, the intermediate core portion andthe ring core portion are expressed as (a,Δ₁), (b,Δ₂) and (c,Δ₃),respectively, conditions of (A) though (D) below are satisfied. (A)0.95≦Δ₁≦1.35; (B) −3.5=V2/V1<0 and 0.5≦3/V1≦4.5 when V1=Δ₁,V2=Δ₂×{(b/a)²−1} and V3=Δ₃×{(c/a)²−(b/a)^(2};) (C) α, expressed withα=−y{x−1}/Δ₁, is such that 0.10≦α≦0.45 when x=C-band y=Δ ₃/Δ₂; and, (D)the value resulting from dividing chromatic dispersion slope at awavelength of 1.55 μm by chromatic dispersion is 0.0025 nm¹ or more and0.0035 nm⁻¹ or less.
 21. A hybrid transmission line comprising adispersion-compensating optical fiber according to claim 16 and asingle-mode optical fiber for transmission wherein, at a wavelength of1.55 μm, said single-mode optical fiber for transmission has aneffective area of 40 μm² or more, positive chromatic dispersion, and acutoff wavelength that substantially allows single-mode propagation. 22.A hybrid transmission line according to claim 21 wherein, at awavelength of 1.55 μm, said single-mode optical fiber for transmissionhas an effective area of 70 μm² or more, and chromatic dispersion of +16ps/nm/km or more and +22 ps/nm/km or less.